Many of the questions I get on this website and the forums are of this type.  People understand the general concept of hormesis, namely that exposure to controlled amounts of stress can be beneficial, because it elicits beneficial adaptive responses in the organism.  They understand that weight lifting builds muscles, and that intermittent fasting and calorie reduction can be healthful. But too much of any stressor — weight lifting, caloric restriction, sunlight, allergens  – can have adverse consequences.  With hormesis, it seems, the Goldilocks principle applies: to get a benefit, the level of stress must be “just right”.  And because it’s so easy to veer into overload, many people seek to avoid even mild stress:  Avoid allergens. Cover up with sunscreen. Eat frequent small meals. Don’t exert yourself. But if you choose this path, you forgo the possible hormetic benefits.

So how do you determine the optimum level and frequency of exposure to a stress?  And how much rest or recovery between exposures is optimal?

These are important questions, difficult to answer with certainty.  Of course, all over the Internet you will find those who tell you exactly how many days each week is optimal for lifting weights, how much sun tanning is safe or dangerous, what level of dietary carbohydrate or food restriction is optimal or unhealthy.  In some cases, they will cite studies to support their position. But there is one big problem with all this advice, even the advice based upon careful scientific studies:

Individual responses to hormetic stressors can vary significantly.

Just as responses differ between individuals, a given individual’s ability to tolerate and benefit from hormesis changes over time, and as a function of previous exposures to stressors.  This makes it virtually impossible to reduce hormesis to a simple formula. And yet, the situation may not be so hopeless.  There are actually some tools and metrics we can use to quantifiably determine whether hormesis is helping or hurting us, and thus to “adjust” the dose.

Allostasis. There is a general biological principle that can help us dial in the right level of hormesis.  The principle is called “allostasis”.  Most people are familiar with the related concept of homeostasis, the tendency of a system to maintain a constant internal state, such as the pH, temperature, or oxygen concentration of the blood, within a fairly narrow range.  This concept was developed by the famous nineteenth century biologist, Claude Bernard, who observed that organisms strive to control their internal environment, or milieu interieur, within tight physiological constraints, through physiological processes that resist disturbances from the external environment and quickly restore normal operating conditions.   This notion was later formalized by Walter Canon as “homeostasis”, the tendency of a biological system to regulate its internal environment within a stable range.

While the concept of homeostasis has some validity, in actuality it is of fairly limited application.  In fact, most biological systems do not self-regulate physiological variables within a narrow range, but tolerate a fairly wide range of variation.  During the course of a typical day, blood glucose and insulin levels rise and fall by as much as 50% or more.  Blood pressure, heart rate, and adrenaline surge upon waking and standing in the morning, and increase to further heights when engaging in vigorous exercise, or responding to threatening or emotional situations.

Bernard and Cannon developed the concept of homeostasis to apply only to regulation of the internal environment, particularly that of the cell or circulatory system. It was not intended to describe the external condition of organs or whole organisms.  Yet others have extrapolated this concept and applied it to the misleading notion of “set points”.  For example, some have advanced the idea that each of us is born with a body weight set point from which we can only deviate transiently and in a futile manner through diet and exercise, but which we are doomed to return to.  But body weight or body fat is not an “internally” regulated physiological variable, despite the efforts of some to tie this to the hormone leptin.  Rather, it is the result of a number of interacting systems, which frequently lead to a relatively stable output.  I’ve provided a more detailed critique of the set point concept in my post, Change your receptors, change your set point.

On the contrary, when you consider the whole organism, you are struck more by its variability over time than by its constancy.  Sterling, Eyer and McEwen have contrasted the stability of homeostasis with what they call “allostasis” or “stability through change”. “Stability” here does not mean a static state, but rather a dynamic physiological process which allows the organism to sustain itself in the face of external challenges.  For example, hormones like cortisol, adrenalin and catecholamines, and mediators like cytokines, allow us to adapt to changes in activity level. Digestive hormones like insulin and glucagon, and secreted digestive enzymes like proteases, amylases and lipases, allow us to effectively respond to the sudden ingestion of food, otherwise known as “meals”.  On longer time scales, major morphological changes in the overall shape and and size of the body allow animals to handle episodic changes like pregnancy, migration, or hibernation.  While organisms and physiology are stable enough to survive, they do not maintain or even strive for a state of constancy.

Allostasis, not homeostatsis, better describes how we deal with changing circumstances.

Changes which are beneficial in the short term to handle an external stress, may be harmful or pathological if maintained chronically.  So for example, glucocorticoid and catecholamine hormones such as cortisol and adrenaline are helpful, even essential, for gearing the body up to handle acute stress.  Without such hormones would we be unable to get up in the morning, much less deal with emergencies. But these same hormones become harmful or deadly when chronically elevated, causing significant damage to the cardiovascular system and neurodegenerative conditions such as depression and memory loss.  The “biphasic” effect of cortisol and other arousal hormones and catecholamines is encapsulated by the Yerkes-Dodson Law, illustrated in the figure at the right, which holds that performance increases with physiological or mental arousal, but only up to a point. When levels of arousal become too high, performance decreases.

Similarly, insulin, which is essential for the short term digestion of carbohydrates and protein, and for facilitating tissue growth, can likewise be harmful if elevated chronically, leading to obesity, cardiovascular disease, inflammatory diseases, and possibly cancer.  McEwen refers to the elevation of these stress related hormones and effectors as “allostatic load” and their chronic elevation as “allostatic overload”.

Hormones are neither good nor bad in and of themselves.  They are helpful at the right time and for the right length of time.

Alternating states and opponent processes.  I’ve written about opponent processes as an explanation for psychological adaption in my post on The opponent-process theory of emotion.  Here I would like to go further and generalize the opponent process theory to more broadly characterize our adaptive physiology.

Our natural allostatic variability typically manifests itself in an oscillation between two states or “extremes” which alternate or fluctuate over some characteristic interval of time that can range from seconds, to hours, days, months, or years.   These two states are often thought of as “high” and “low” levels of some variable hormone, enzyme or effector. But I think they are better considered merely as opposing conditions.  That’s because what appears to be “states” are really the results of underlying processes that move the organism in opposite directions — opponent processes. These processes typically come in pairs and act to balance each other, like yin and yang. It is important not to confuse the states and the opponent processes.  These alternating states are the resultant outcomes of the opponent processes; the visible “state” reflects the dominant process, but both processes are always in play to greater or lesser extents.

This concept of may be confusing, so here are a few examples of alternating states and associated opponent processes, with widely varying temporal scales. In each case “State A” exists when “Process a” dominates over “Process b”, and “State B” exists when process b dominates:

State A     State B            Process a        Process b                     Frequency

Eating      Fasting             Anabolism       Catabolism                    3-24 hrs
Waking    Sleeping           “C” process      ”S” process                   24 hrs
Exercise   Rest                Sympathetic     Parasympathetic            varies

Eating and fasting. You could attempt to characterize the A and B states as “active vs. passive”, “stressful vs. restful” or “bad vs. good” but that is not quite right. Take eating and fasting, for example.  You might argue that eating is the active or stressful state, because it places a demand on the digestive system, and the fasting period between meals allows the digestive system to recover.  However, if the fast is continued beyond a certain point, it becomes the stressor.  After about 12 hours, the stress of fasting causes a rise in catabolic “breakdown” processes, upregulates the neuroprotective hormone BDNF, and the process of autophagy activates the breakdown of intracellular materials to fuel the mitochondria. Utilized in moderation, the “stress” of fasting thereby activates beneficial processes that protect and defend us.  Once you resume eating, the “stress” of fasting is relieved and the anabolic “building” process kick in with the rise of insulin.  This has its own benefits, in repair and growth.  It is important to note that the anabolic hormones like insulin and the catabolic ones like glucagon or adrenaline are always present at some level; they never “go to zero”.  Yet one or the other is dominant at a given time, depending on the state of digestion.

Wake and sleep. Similarly, you could say that wakefulness is active and stressful, whereas sleep is passive and restorative.  But again, this would be misleading. Wakefulness and sleep are the outcome of a dynamic, alternating balance between two essential processes, the “C process” and the “S process”. The “C process” generates a wakeful state based upon activation of  the ascending arousal system, including cholinergic, noradrenergic, serotoninergic, dopaminergic, and histaminergic neurons located in the hypothalamus and other brain nuclei.  These neurons release corticotropin-releasing factor (CRF),  ACTH, and cortiosol on a regular diurnal cycle. This arousal system interacts with inhibitory “sleep-active” neurons in the ventrolateral preoptic nucleus (VLPO), releasing GABA and other sleep-inducing neurotransmitters.   These sleep promoting neutrons and neurotransmitters represent the “S” process. The result is a “flip-flop switch”  producing distinct sleep and wake states with abrupt transitions.  The “C” and “S” processes each never actually stop, but they continuously wax and wane, with one of the two becoming dominant and leading to either wakefulness and sleep. Even within the states of wakefulness and sleep there are many regular oscillating subcycles; for example REM sleep, deep sleep and light sleep. Disruptions in this process can lead to insomnia, and can be corrected by Sleep Restriction Therapy, as I’ve described in my post A cure for insomnia.

The reality is that for each basic physiological process we need both A and B states and the underlying a and b processes.  The opponent processes represent polarities of an indivisible “yin-yang” pair.  They balance each other, but not in a constant ratio.  The a and b processes cannot be indefinitely sustained, but each have within themselves the seeds of their own demise, by inducing their complementary, inhibitory process.  Biological organisms are constructed out of complementary and opposing physiological process, which naturally give rise to  an alternation between the A and B states.  This is a phenomenon I will refer to as stress oscillation.

Stress oscillation builds dynamic range.  So what does allostatis and the opponent processes have to do with hormesis?  Sometimes hormesis is thought of unidimensionally:  lift weights to build muscle.  Fast or reduce carbohydrates to lower insulin and reduce weight.

But in reality, hormesis should be thought of as a binary process of alternating stress and recovery.

Lifting weight builds muscles because it induces “catabolic” microtrauma to the muscles; it is the rest between workouts, in combination with adequate diet, that leads to the “anabolic” rebuilding of the muscle.  Both stress and recovery are necessary.  For the same reasons, weight loss through insulin lowering should be balanced with sufficient periodic insulin raising to maintain lean body mass, and maintain the healthy function of the insulin producing system, including the pancreatic secretory islets and the insulin receptors in the brain and muscle tissues.  One risk of an unremitting “insulin sparing” diet, such as a very low carbohydrate diet without periodic insulinogenesis is the induction of a state of physiological insulin resistance. This is indeed a paradoxical outcome of a diet which many pursue in order to improve their insulin sensitivity!

In the wake-sleep cycle, the ascending arousal system or “C-process” is stimulated by the secretion of CRF (corticopin releasing factor) by the hypothalamic-pituitary-adrenal (HPA) axis.  But a state of interminable wakefulness or insomnia results in cognitive deterioration. Both the “C” and “S” processes are necessary, and they must oscillate:  An unvarying simultaneous activation of both processes would not lead to cognitive stability, but rather mental deterioration.  Stress and renewal must follow one another as night follows day.

For any physiological function like digestion, muscle synthesis, or the wake-sleep cycle, the oscillation between State A and State B produces a dynamic stability that exhibits a certain dynamic range between stress and rest.   The cycle of eat-fast-eat leads to a cycling of digestive hormones such as insulin, glucagon, and adrenaline.  The cycle of wake-sleep-fast leads to a cycling between the arousal system and the sleep system.

And here is the takeway:  By exposing ourselves to alternating A and B states of increasing intensity, we build tolerance and dynamic range for the opponent processes.  We should strive to increase the magnitude of contrast between the opponent states.  I believe that we can generalize the use of dynamic capacity between allostatic states as a marker of fitness.  This can be illustrated by several examples:

Example 1.  Aerobic capacity.  Exercise phyiologists understand that athletes are able to build aerobic capacity (so-called VO2 max) by exerting themselves at or near maximal heart rate.  Their state of fitness is manifest in a reduced resting heart rate or pulse, and a higher ratio between peak VO2 and resting VO2.  This ratio or difference is sometimes referred to as VO2 reserve or VO2R, and it represents a good measure of aerobic fitness, a kind of dynamic capacity to oscillate between rest and exertion. Yet another measure of dynamic capacity is the rate at which heart rate or VO2 return to normal, after exertion

What is interesting is that training harder does not necessarily increase VO2R or dynamic capacity.  A study by the Navy Seals showed that overtraining can actually decrease VO2R, and can elevated resting heart rate by as much as 10-15 beats per minute.  Monitoring your resting heart rate is an excellent way to know if you are overtraining.  (Caveat: the heart rate measure must be used with judgement, as severe overtraining can lead to extreme exhaustion and an abnormally low heart rate).

More generally, high intensity interval training (HIIT), whether it be in the form of weight lifting, sprinting, or other metabolic training, is based on the very same premise.  Maximal exertion, into the anaerobic range, activates the full range of muscle fibers, including the ever-important fast-twitch muscle fibers, empties muscle glycogen, and activates the glycolytic pathway, resulting in an upregulation of insulin receptors (GLUT4 transporters), and improved insulin sensitivity.  But for HIIT to work effectively, it is equally important to allow adequate time for rest and recovery.  (I’ve discussed this in more detail on the Fitness page of this blog, with particular emphasis on the physiological analysis of Doug McGuff in his book, Body by Science).

For sports as varied as running and weight lifting, the well known principle of periodization recognizes the importance of variation in intensity and proper rest. In short, both high intensity training and aerobic training, if carried out with adequate rest and recovery, build dynamic range.

Example 2.  Digestive or metabolic fitness can be measured by a low basal insulin level in combination with a pattern of sharp, but brief insulin secretion in response to ingested carbohydrates or insulinogenic protein.  Low basal insulin level is seen, for example in non-industrialized populations such as the Kitavins, whose average basal insulin levels of about 4 mIU/ml are about half those of Western populations.  And yet the Kitavans consume meals with a high percentage of carbohydrates and have good insulin sensitivity.   So low basal insulin levels alone are not the whole story. The optimal pattern seems to involve an alternation between feast and fast, allowing the digestive hormones and enzymes to cycle between anabolic (insulin) and catabolic (glucagon, adrenaline, and cortisol).

This is also the premise behind the concept of intermittent fasting.  By training yourself to cut out snacks and go for longer periods of time between meals, the metabolic system — which includes not only digestive hormones and enzymes, but neurotransmitters and hypothalamic receptors — adapts to increase its dynamic capacity.  The resulting benefits are lower basal levels of anabolic hormones like insulin and catabolic hormones like glucagon and adrenaline. But just as importantly, intermittent fasting develops improved sensitivity and the ability to both ramp up and reduce these hormones quickly and responsively.

The benefits of spending time in the fasting state are numerous, including a natural detoxification and nutrient recycling process known as autophagy, and the upregulation of brain-protective growth factors such as Brain-Derived Neurotrophic Factor (BDNF).  Fasting allows for the upregulation of fat-liberating enzymes and hormones and a significant and glucose transporters, thereby improving insulin sensitivity. McEwen has compiled research showing that an appropriate level of “stress” or allostatic load will increase markers of brain plasticity. By contrast, following the conventional wisdom to eat six small meals a day of controlled glycemic foods, in the misguided attempt to “regulate” blood glucose at a constant level, deprives your body of these important restorative and protective processes.

But at the other extreme, extensive fasting or strict low carbohydrate dieting can leave the pancreas underutilized and thereby lead to a reduction in glucose transporters in the cells, since these are no longer “demanded”.  Our cells and organs tend to “economize” by synthesizing only the machinery they really need: use it or lose it.  People who abstain from or never consume milk will lose the ability to produce the enzyme lactase, so they become lactose intolerant.  Similarly, we need to regularly “exercise” our ability to secrete insulin on demand and the ability of cells to utilize glucose. This doesn’t necessarily have to occur every day, but several glucose loadings a week are probably necessary.

So the wise course is to apply “stress oscillation” to diet, and alternate judiciously between fasting and nutritious, balanced meals with a variety of macronutients and micronutrients.   Remember that the “stress” is binary: fasting represents recovery from the “stress” of eating; and eating relieves the “stress” of fasting.   A dynamic approach of hormesis involves stretching the ability to move between these two poles, increasing “allostatic capacity”.

Example 3.  Stress, health and cortisol.  Of all the hormones, cortisol has acquired a reputation as “the bad guy”.  It is well known that elevated cortisol levels are the mark of chronic stress and adrenal fatigue.  It has been suggested that higher levels of cortisol are linked to disregulated or high blood glucose levels and predispose one to diabetes. Chronically elevated cortisol also damages neurons in the hippocampus, leading to memory loss and cognitive decline. As a result, some practitioners mistakenly advise trying to minimize stress and even eat frequent meals, in order to keep cortisol at bay and avoid “stressing” the adrenal glands. But this is a one-sided perspetive.  Cortisol is necessary to normal alertness and mental function, as well as our ability to respond to sudden demands like exercise or threats. The problem comes when cortisol does not exhibit a normal morning peak level, followed by a steady decline through the day, but instead remains flat or even increases in the evening.  Chinook et al. classified four different cortisol patterns, shown below.  Pattern 1 (Graph A) is normal; Patterns 2, 3 and 4 show the flattening or later peaks that characterize dysregulation:

Diurnal or event-related elevations in cortisol are not problematic, so long as cortisol levels return to baseline at a decent rate, as in Pattern 1. According to Lovell et al., higher percieved stress levels are reflected not so much in average cortisol levels, but rather as higher basal or evening cortisol levels, and flatter diurnal fluctuations in cortisol levels. Matthews et al found that individuals with the flattest cortisol pattern (slowest rate of decline to baseline) were most at risk of coronary calcification.  Sephton et al found that flatter cortisol patterns were predictive of suppressed immunity and lower survival rates in women with metastatic breast cancer.

In short, we should be less concerned with absolute cortisol levels, than with the pattern of cortisol secretion.  As with other hormones, increased dynamic range and a robust cyclical pattern are indicative of fitness, stress-hardiness, and health.

The larger lesson.  James Loehr (about whom I wrote in my earlier post on Stress management and toughness training) has written eloquently about the use of “stress oscillation” to build athletic capicity and resilience in the corporate world in his book The Power of Full Engagement:

Balancing stress and recovery is critical not just in competitive sports, but also in managing energy in all facets of our lives. When we expend energy, we draw down our reservoir. When we recover energy, we fill it back up.  Too much energy expenditure without sufficient recovery eventually leads to burnout and breakdown…Too much recovery without sufficient stress leads to atrophy and weakness….Oscillation occurs even at the most basic levels of our being. Healthy patterns of activity and rest lie at the heart of our capacity for full engagement, maximum performance, and sustained health. Linearity, by contrast, ultimately leads to dysfunction and death. (TPOFE, pp. 29-31).

How to apply stress oscillation to your life. Let’s return to the question at the beginning of this post: How much of any kind of stress is enough, but not too much, to generate a hormetic benefit? The answer is: This is the wrong question!  You should not be striving for some magic optimum level of constant stress. Rather, you should strive to oscillate stress, by exposing yourself to intermittent, but intense sources of stress.  This builds dynamic capacity or strength. The amount and frequency of the stress are variables you can experiment with, but younow have a way to measure the benefit and know whether you are on track. The key metric is dynamic capacity. The appropriate measures of dynamic capacity depend upon what our goals are:

• For physical fitness:  a high VO2 max during exertion combined with a low resting VO2, resting pulse, and blood pressure.
• For dietary or metabolic health:  a rapid insulin and blood glucose response to food and low basal insulin and blood glucose levels
• For stress hardiness:  peak cortisol levels upon waking, followed by steady decline to low evening (basal) levels.

These may be imperfect measures, and they are subject to exceptions and interpretations based upon special health circumstances. Some of these measures are easy to implement at home; others are less convenient because they require blood or saliva analysis (which can be purchased online). But the general principle is valid:  Don’t look for average biometric values, but look for the dynamic range between high and low. And look for an oscillatory pattern that demonstrates periods of testing and building capacity, alternating with periods of rest and recovery.  I’ve discussed only three applications here in detail: digestion, exercise, and general stress tolerance.  But the principle of stress oscillation can be applied to many other applications of hormesis:  suntanning, allergen immunotherapy, cold showers or plus lens therapy.  I leave it to the curious reader to think about the physiological processes at work, and the appropriate measures of improved dynamic capacity.

The goal of hormetic stress should be to increase dynamic capacity to handle allostatic load — variable stresses — in a measureable way.  The precise level and frequency of stress exposure will vary from person to person. This is not a one-size-fits all path to health, but rather a journey that each of us must take for ourselves.  But on this journey, our engine is stress oscillation and our compass is increased dynamic capacity.

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Consider the following ten situations:

1. Drug addicts, before becoming addicted, experience the euphoria of a drug with few negative consequences. Over time, however, they develop a tolerance for the drug, requiring increasing doses to get the same high.  At the same time, their cravings and distressful feelings increase when going without the drug, leading to increased in withdrawal symptoms and a cycle of increasing drug use.
2. Firefighters and emergency room doctors have stressful jobs, but many find themselves experiencing an irresistible rush and heart-throbbing exhilaration from these fast-paced occupations.
3. New lovers, after a honeymoon period of initial infatuation, often experience a drop-off in affection, leading to dissatisfaction, fights, and sometimes breakups.  When reconciling after the breakup, they experience renewed closeness for a period of time. Typically, the more intense the infatuation, the greater the strife and negativity during the falling out periods.
4. Marathoners and other runners often experience a “runner’s high” which builds up during longer, more strenuous runs, and can extend for hours or even days after a run. Runner’s high has been associated with release of endorphins, a natural “opiate” produced by the body.
5. Infants who are given a bottle and start sucking on it experience pleasure.  But if the bottles are removed before the infants have finished feeding, they universally cry.  And yet they would not have cried if the bottle had never been given.
6. Depressed adolescents often resort to “cutting”, a form of self-mutilation that introduces some pleasure or even a high into their otherwise sad or pleasureless day.  They often find the need to increase the cutting to maintain the pleasure.
7. Scratching an itch generally relieves the itch and can be pleasurable, but often this ends up making the itch more intense and, after repeated itching, even painful.
8. Horror movies, which initially are disturbing or even terrifying, can become addictive
9. Politicians and executives in positions of power come to crave the power.  When they are out of the limelight, they experience a letdown, boredom, or even depression.  Upon retirement, this depression can lead to poor health or shortened longevity.
10. People who donate blood frequently report a sense of well being and pleasure that cannot be explained in terms of the blood removal itself.

Can you see the pattern?  In the odd-numbered examples above, pleasure turns to pain; in the even numbered examples, pain becomes pleasure. And in all cases, the effect intensifies with repetition. But why does this occur?  One possible explanation for these types of situation is described in William Irvine in his book “A Guide to the Good Life”:

The psychologists Shane Frederick and George Loewenstein have studied this phenomenon and given it a name: hedonic adaptation. To illustrate the adaptation process, they point to studies of lottery winners. Winning a lottery ticket typically allows someone to live the life of his dreams. It turns out, though, that after an initial period of exhilaration, lottery winners end up about as happy as they previously were. They start taking their new Ferrari and mansion for granted, the way they previously took their rusted-out pickup and cramped apartment for granted. (Irvine, p. 66).

Hedonic adaptation is the experience of “getting used to” a good or pleasurable thing until one returns to a state of relative indifference or equilibrium, feeling about the same as one did beforehand. As I describe in more detail on the Stoicism page of this blog, Irvine goes on to point out how the Greek and Roman Stoics were able to combat hedonic adaptation by practicing techniques such as “negative visualization”, in which they regularly took time to vividly imagine loss of people, relationships and possessions they held dear, so they could better appreciate what they had.

Hedonic reversal and habituation. While hedonic adaptation of this sort certainly exists, the ten situations I listed above are quite different than than that of the lottery winner that Irvine describes. My ten situations do not involve a return to homeostasis or equilibrium. They involve a total switch, what I will call hedonic reversal. Pleasure becomes pain; pain turns to pleasure. This is the phenomenon that Richard Solomon tries to explain in his paper.  Solomon quotes Plato, who may been the first to describe true hedonic reversal and puzzle over it:

How strange would appear to be this thing that men call pleasure! And how curiously it is related to what is thought to be its opposite, pain! The two will never be found together in a man, and yet if you seek the one and obtain it, you are almost bound always to get the other as well, just as though they were both attached to one and the same head….Wherever the one is found, the other follows up behind. So, in my case, since I had pain in my leg as a result of the fetters, pleasure seems to have come to follow it up.

In hedonic reversal, a stimulus that initially causes a pleasant or unpleasant response does not just dissipate or fade away, as Irvine describes, but rather the initial feeling leads to an opposite secondary emotion or sensation. Remarkably, the secondary reaction is often deeper or longer lasting than the initial reaction.  And what is more, when the stimulus is repeated many times, the initial response becomes weaker and the secondary response becomes stronger and lasts longer. This is what happens quite clearly in the case of addiction. After repeated administration, the original dose no longer gives the same high, so it must be increased to achieve that effect. In addition, as time goes on, abstaining from the addictive dose becomes more difficult, while cravings, anxiety and depressive feelings increase. The mirror image of this addictive pattern is apparent in the case of endorphin-producing athletic activities like running, or thrill-seeking pasttimes like parachuting. Solomon reports on a study of the emotional reactions of military parachutists:

During the first free-fall, before the parachute opens, military parachutists may experience terror: They may yell, pupils dilated, eyes bulging, bodies curled forward and stiff, heart racing and breathing irregular. After they land safely, they may walk around with a stunned and stony-faced expression for a few minutes, and then they usually smile, chatter, and gesticulate, being very socially active and appearing to be elated….The after-reaction appears to last about 10 minutes…After many parachute jumps, the signs of affective habituation are clear, and the fearful reaction is usually undetectable. Instead, the parachutists look tense, eager or excited, and during the free-fall they experience a “thrill”. After a safe landing, there is evidence of a withdrawal syndrome. The activity level is very high, with leaping, shouting…and general euphoria. This period, often described as exhilaration, decreases slowly in time, but often lasts for 2-3 hours. Indeed, I was once told by a sport parachutist…that his “high” lasted 8 hours. A new, positive source of reinforcement is now available, one that could never have eventuated without repeated self-exposures to an initially frightening situation to which the subject then becomes accustomed. (Solomon, pp. 693-8)

Thus, both the addictive pattern and the thrill pattern share the features of hedonic habituation (reduced intensity of the primary response) and hedonic withdrawal (heightened intensity of the secondary, opposite response). In surveying and studying a wide range of such experiences, Solomon found a common pattern of hedonic contrast, which he represented as follows:

baseline state → State A → State B

State A is the initial emotional or “affective” response to a stimulus, which can be either pleasant or unpleasant.  Typically, the first time a novel stimulus is applied, the primary or State A response is most pronounced at the outset and then tapers to steady level as long as the stimulus is maintained, as shown below in Figure 4.  For example, exposure to the heat of a sauna or hot tub may cause an initially hot or burning sensation, which diminishes somewhat over time. Once the stimulus is removed, the sensation is replaced by a contrasting sensation or affective state, the after-reaction, or State B.  State B is opposite in hedonic character to State A. If one is pleasant, the other is unpleasant, and vice versa. Initially, and after the first few stimulations, State B typically has a much lower intensity than State A, but often lasts longer in duration, before it eventually decays and returns to the baseline state.

What Solomon noticed is that after many repeated stimulations, the intensity of State A typically diminishes, both in peak intensity and steady state intensity. This is the hedonic habituation effect, also called “tolerance”, and it is seen with both pleasant and unpleasant affective reactions. The only way to increase the intensity of State A is to increase the magnitude of the stimulus. At the same time, with repeated exposures, the secondary affective State B often intensifies and lasts longer. This is the hedonic withdrawal effect. This combination of habituation and withdrawal effects is shown in Figure 5:  For addictions, the pleasurability of the stimulus diminishes with time and the unpleasant withdrawal grows in both intensity and duration. For the thrill-seeking or excitatory pattern, the stressfulness or unpleasantness of the stimulus is reduced with repetition, while the  ”withdrawal” becomes more pleasant and lasts longer, before returning to baseline.

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The opponent-process theory. So far, all we have presented is a qualitative description of some common patterns of sensory or emotional response, without any real explanation for why these patterns occur as they do. But Solomon’s real innovation is that he can explain these patterns by decomposing them into more elemental underlying biological processes. His central insight is that the nervous system is organized in such a way that any sensory or emotional response can be decomposed into two concurrent processes. The State A response diagrammed in Figures 4 and 5 above is in reality a composite of two complementary physiological processes:

• a primary process “a”, which is the direct observable response to the stimulus; and
• an opponent process “b”, which acts to inhibit or counteract the primary process.  It occurs at the same time as the primary process, but is not always evident or easy to perceive.

To understand how these processes actually work in practice, let’s look more closely at Figure 7 below. The opponent process “b” actually begins shortly after the initiation of the primary process “a” and acts to dampen it during what we observe as State A. Because “b” is both smaller and opposite in effect to “a”, it acts to reduce the net impact of “a”.  That explains why the intensity of the A process is greatest at the outset, but drops as the stimulus in continued.   According to Solomon, for a novel stimulus the “b” process is smaller and more sluggish than the “a” process.  It is slower to built to its steady state level (asymptote) and slower to decay after the stimulus stops.  This is shown in Panel A of Figure 7:

So what happens to bring about habituation after many repetitions of the stimulus, when the stimulus is no longer novel? According to Solomon, the primary “a” process remains unchanged in response to the stimulus.  What changes with repetition is the opponent process “b”.  As depicted in Panel B of Figure 7, after many stimulations:

• it intensifies
• it starts earlier (reduced latency period)
• it decays more slowly

The net impact of these changes in the opponent process is to progressively dampen the magnitude of State A and increase the speed, magnitude and duration of State B.  Thus, without any changes in the primary process, these changes in the opponent process can fully explain the increase in both tolerance and withdrawal, as shown in Figure 7.

Biological basis. Opponent processes are not just some clever hypothetical construct that Solomon came up with out of thin air. These kinds of inhibitory processes are common in biological systems.  For example, many or perhaps most neurotransmitters, hormones, and biological receptors have corresponding opposites, which act to inhibit or moderate the primary response. These inhibitory processes serve a useful biological control functions by preventing over-reactions to environmental disturbances. They form the the biological basis of systems of homeostasis, systems that enable organisms to resist or adapt to disturbances to their steady functioning.

Solomon’s opponent-process theory also identifies several key factors that can strengthen or weaken the opponent “b” process.  His paper summarizes some very clever animal research on distress behavior in ducklings, from which he deduced that the opponent process can be strengthened in three primary ways:

• increasing the intensity of the initial stimulus exposure
• increasing the duration of the stimulus
• shortening the interstimulus interval (the time between stimulus exposures)

Interestingly, merely repeating the stimulus, in and of itself, had no effect on strengthening of the opponent process if the stimulus was too weak or too short, or if the interstimulus interval was too long.  In particular, he found that, depending on the inherent duration of the opponent process, the interstimulus interval had a major effect on whether or not the opponent process will increase in strength.  According to Solomon

The critical decay duration is that disuse time just adequate to allow the weakening of the opponent process to its original, innate reaction level. If reinforcing stimuli are presented at interstimulus intervals greater than the decay duration, then the opponent process will fail to grow. (Solomon, p. 703)

Each opponent process has an inherent decay behavior, that is, a rate at which it fades away.  This will depend on the specific physiological and biological underpinnings of that process.  On a biochemical level, for example, this decay duration may depend on the half-life of the neurotransmitters, hormones, or receptor behavior involved.  It will surely also involve higher order processes which relate to the nervous system and psychological conditioning of the individual.  Figuring out the decay duration of various opponent processes should be a matter open to empirical determination.  It can be approached both by psychological investigations on others (or on oneself), and also by looking into the underlying physiological and biochemical mechanisms.

The final element of Solomon’s theory is a phenomenon he calls “savings”.  Although opponent processes can be weakened or faded away by avoiding the stimulus for an extended period of time, that does not mean they leave no memory traces. Studies show that these opponent processes are more quickly reactivated the next time they are re-stimulated. Reflexes and emotional reactions build up more quickly when reactivated than they did with the initial stimulation. According to Solomon,

Such a phenomenon is not unexpected. In alcohol addiction, for example, the abstainer is warned that one drink may be disastrous, and the reason is the savings principle. The reexercise of alcohol’s opponent-process system strengthens the withdrawal syndrome very rapidly and sets up the special conditions for resumption of the addictive cycle. Cigarette smokers report the same phenomenon: Readdiction to nicotine takes place much more rapidly than does the initial addiction. (Solomon, p. 703)

This savings effect also applies to positive opponent effects, such as the exhilaration experienced by skydivers or runners when resuming their thrilling or strenuous activities after a hiatus.  Understanding this effect is important in designing strategies for avoiding or minimizing the negative effects of relapse, as will be discussed below.

Put into simplest terms, the opponent-process theory explains the psychology of addiction and thrill-seeking in terms of the strengthening of inhibitory processes.  These inhibitory processes  get stronger when stimulation of a primary emotional response is sufficiently intense, sustained and frequent.  They become evident only when there stimulus and the primary processes are not present, and typically last for some time afterwards.   On subsequent re-exposure the stimulus, opponent processes often reactivated more quickly.

Is this a biologically realistic explanation?  Perhaps Solomon has not generated a broad enough set of hard physiological data to conclusively prove his hypothesis.  However, there is still a strong case in favor of it. First, his hypothesis provides a model which offers a coherent and consistent explanation for a wide range of  sensory and emotional behaviors for which there are few other good explanations. Second, there one application of the Opponent-Process theory–to an area unrelated to emotions–which has already been empirically verified:  the explanation of color perception. It is worth spending a paragraph on this because it provides some insights into the biological reality of this theory.

The opponent-process theory of color vision. Until the late nineteenth century, the primary theory of color vision was the trichromatic theory, which held that color perception was the result of the stimulation of three different types of cone receptors in the retina of the eye.  In 1892, Ewald Hering first proposed the opponent-process theory of color vision. He observed that any color can be uniquely analyzed in terms of the colors red, yellow, green, and blue, and noted that these four primary colors exist as the complementary pairs red-green and yellow-blue. Hering’s theory accounts for how the brain receives signals from different kinds of cone cells and processes and combines these signals in real time. The opponent-process theory of color vision received further support in 1957 in studies by Hurvich and Jameson, and in 2006 by Liapidevskii. Some of the most compelling evidence for the theory is the phenomenon of complementary color after-images, which cannot be explained by the tricolor theory.  You can demonstrate this for yourself by staring at the red dot in the middle of the image below for 30 seconds without letting your eyes drift from the center; then look at a blank white sheet and you will see the image with a more familiar set of colors. (It may take a while for the image to develop).

Looking at the colors under bright light and for longer periods enhances the opponent (inhibitory) processes in the receptors, which intensifies the after-images, just as one would predict based on the principles Solomon found for sensation and emotion.

Consider the similarity between this contrasting after-image response to visual stimuli and the emotional or affective responses that that Solomon found in his animal studies.  The sensory after-images may be less intense and of shorter duration, but the principle is the same, and both phenomena illustrate how opponent processes can arise within our nervous systems. Beyond the processing of simple nerve signals, such as those involved in visual sensory perception, the opponent process theory can account for psychological processes of increasing complexity and at multiple levels, based on the well established fact that the brain is able to integrate sensory information by adding and subtracting different excitatory and inhibitory inputs from different receptors and neurotransmitters.

Practical applications.  Besides explaining common sensory and emotional reactions, I believe the opponent-process provides some very practical guidance for how we can use pleasant and unpleasant experiences to our advantage.  This guidance can be boiled down to seven basic insights:

1. Be aware of hidden processes! The most important insight is to be aware that any primary sensory or emotional stimulus, whether pleasurable or unpleasant, will give rise to opponent processes of an contrasting nature.  Even though you most likely cannot directly perceive them, these opponent processes are happening–and even growing in strength–at the very same time as the primary emotions and sensations that you do perceive.  When the primary emotions and sensations stop or pause, these contrasting processes emerge into consciousness!  For example if you put your hand in cold water, a “warm” opponent processes is being stimulated, but you feel that warmth only once you withdraw your hand from the water. And the pleasure of overindulging in sweet desserts is likely to be followed by an unpleasant reaction that arises some time after you stop eating.
2. Avoid overexposure to pleasurable stimuli. This does not mean that you should minimize or avoid direct pleasure! Just be aware that too much of a good thing too often can backfire — and be aware WHY that is so. By remaining vigilant, you need only to moderate the intensity and frequency of pleasant stimuli to ensure that the opponent processes do not build up. For example, eating small portions of delicious foods, and spacing out meals — or even individual bites — will tend to reduce the level the opponent processes (cravings) that would otherwise reinforce appetite and cravings. When you go for that second cup of coffee, you may marginally increase your alertness in the short term, but realize that you are at the same time continuing to stimulate a reactive opponent process, counteracting the caffeine high, that may lead to increased tiredness later on.  There is a biological argument for moderation!
3. Use unpleasant and stressful stimuli to indirectly build pleasure. This is one of the most powerful insights of the opponent-process theory. By judiciously exposing ourselves to intermittent stresses, of sufficient intensity and frequency, we activate in our bodies and psyches some powerful opponent processes, which in turn result in heightened pleasure and satisfaction. Depending on the type of stimulus, these indirect pleasures can be short-lived or more sustained. Stressful or unpleasant stimuli can therefore be thought of as a form of “psychological hormesis”:  The nervous systems is activating certain pleasurable inhibitory processes in order to defend against and build tolerance to stress. These pleasure-generating defense mechanisms are real, biological processes which operate in our nervous systems. One well known example is the production of endorphins, our natural opiates, which can be produced by engaging in strenuous exercise. Endorphins literally help us to endure the pain of exercise by providing a counteracting pleasure. So by increasing the intensity and frequency of stress exposures, we are not just building tolerance–we are actively building up a sustained background “tone” of pleasurable emotions. This is very much in line with what the Stoics called “tranquility”. As explained on the Stoicism page, Stoic tranquility is not apathy or a lack of feeling!  On the contrary, it is a positive sense of equanimity, contentment, and happiness that endures and supports us.  It is the opposite of depression; you might even call it “elevation”.
4. Indirect pleasure is superior to direct pleasure. So we have learned that we can paradoxically use pain or discomfort to indirectly cause pleasure.  But is there any reason to think that the pleasure resulting from running, hard work, cold showers, or skydiving is superior to the pleasure from sweet desserts or scratching an itch? Aren’t they equivalent? Doesn’t any pleasure, whether direct or indirect, nevertheless have the potential to lead to addiction?  This is an interesting question, but I think the opponent-process theory makes the case that indirect pleasures — those that results as reactions to stress — are superior. There are two main reasons for this:  First, according to Solomon, opponent-processes are “sluggish”; they take time to build, and decay more slowly. They continue even when the stimulus stops. And unlike direct pleasures, which may be more intense, there is no sudden withdrawal reaction when they stop, hence no “craving”. They tend to fade slowly. Second, the initial unpleasant stimulus — exercise, work, cold sensations — must be sufficiently unpleasant to be effective. This initial unpleasantness will always be a “barrier” that requires conscious effort to face and overcome. If it starts to become “addictive”, it is easier to let this unpleasant barrier stand in the way. It is easy to decide not to go running or take a cold shower if one becomes concerned it is becoming too habit-forming or detrimental to one’s health.
5. Use unpleasant stimuli to counteract addictive pleasures. This is one of the most interesting, and I think unexplored, applications of the opponent-process theory. Addictions are characterized by increased cravings. These arise when opponent process build up in reaction to pleasurable primary stimuli that are too intense and frequent. The craving can become a sustained background “tone” that is always there when the pleasurable stimulus is absent. And the “savings” effect makes the opponent cravings come back more easily. But we can overpower these cravings by deliberately introducing unpleasant stimuli at the same time as the addictive cravings, in order to generate new pleasurable opponent processes. The key is to time the unpleasant stimuli to coincide with cravings or withdrawal, and make them sufficiently intense and frequent, that one builds up sufficient background pleasure tone to counteract the unpleasant anxiety that typically accompanies addictions. So fight cravings by adding a new stressful activity like high intensity exercise, cold showers, or intermittent fasting! It may also help explain why cue exposure therapy — exposing oneself to the forbidden fruit without partaking — can often be more effective in extinguishing addictions than merely abstaining or avoiding the addictive stimulus. It is possible that active cue exposure might generate a type of acute “stress” that “burns out “the original craving with an opposing pleasure. This is like fighting fire with fire!
6. Don’t abuse pain and stress. Despite the potential benefits of controlled stress and unpleasant stimuli to indirectly induce sustained pleasure or “elevation”, this approach is easy to misinterpret or apply incorrectly. Some might take this to be a justification for masochism or self-harm, but it is not. The key here is to carefully think through the consequences of one’s actions. Does the application of the stress or unpleasantness result in an objective strengthening of your body and mind — or does it lead to physical or psychological harm?  Depressed teens sometimes engage in a practice called “cutting” to relieve their depression and apathy, because it can actually reactivate pleasure or a rush that fills a gap and can become addictive. Most likely, this pleasure can be explained in terms of opponent processes that release some of the same endorphins or other neurotransmitters as exercise does. But one needs to distinguish between objectively harmful activities like cutting and beneficial habits like exercise or cold showers. Far from injuring oneself, these beneficial uses of stress and “pain” act to act to build strength, resilience, and long-term happiness.
7. Optimize your stimulation schedule. Be aware of critical decay durations and savings effects of opponent processes, for both pleasant and unpleasant stimuli. Addictions and cravings can be minimized by reducing the frequency of exposure to pleasure-triggers to allow enough time for any cravings to decay. The next time you are mindlessly wolfing down bite after bite of an addictive snack like popcorn or candy, try spacing out bites to allow the craving sensations to die off between bites and see whether you end up satisfied with fewer bites. On the flip side, if you are finding it hard to get started on a healthy habit like strenuous exercise, cold showers, or fasting, it may be that you need to increase the frequency and intensity of the new habit until it takes. According to Solomon, it will become increasingly pleasant if you do this.

Since becoming aware of the opponent-process theory, I applied it to myself in two instances recently:

• On the pleasure side, I reduced my craving for alcohol by drinking less frequently, and limiting the amount that I drink.   The pleasure remains, but the daily cravings have disappeared. I’ve documented this on the Discussion Forum of this blog.
• On the pain side, I have increased my enjoyment of cold showers by never missing a day, by lengthening the showers, and by making sure to expose my most sensitive body parts to the coldness.  This has significantly increased the pleasure I feel, and it comes on more quickly while in the shower (within 10-15 seconds, versus previously more than a minute) and the warm, exhilarating post-shower feeling lasts all morning.  I’m happy all the time, and I definitely feel less stress.

Think about how this might apply to your own situation. Are there pleasures in your life that tend to result in cravings when they are absent? Can you think of ways to introduce healthful but somewhat unpleasant stress into your life in a way that builds your resilience and at the same time a deeper level of satisfaction and sustained pleasure?  Can you use this indirect pleasure to displace cravings or dissatisfaction? And in both cases, how aware are you of the relationship between the intensity and frequency of the stimuli, and the tendency to foster opposing processes that turn pleasures into pains, and pains into pleasures?

The potential applications are infinite!