Biology 202
1998 Third Web Reports
On Serendip

Seasonal Affective Disorder: A Clear Link Between the
Outside and the Inside of the Brain

Julia Johnson

And God said, Let there be light; And there was light.
And God saw that the light was good;
And God separated the light from the darkness.
God called the light Day, and the darkness he called Night.
And there was evening and there was morning, one day.
(Genesis 1:3-5) (1)

The sun has been an endless source of inspiration, both physical and spiritual, throughout the ages. For its light, warmth, and the essential role it has played in the maintenance of the fragile balance of life on earth, the sun has been honored and celebrated in most of the world's religions. While the regeneration of light is constant, the relative length of time between the rising and setting of the sun is affected by the changing of the seasons. Hippocrates postulated centuries ago that these changing patterns of light and dark might cause mood changes (9). Seasonal downward mood changes of late fall and winter have been the subject of many sorrowful turn-of-the-century poems of lost love and empty souls. For some, however, “the relationship between darkness and despair is more than metaphoric (6).

It is only recently that the distinct neurological effects associated with the shortening of the day in winter have been more clearly understood. One interesting example of mood change associated with seasonal change is Seasonal Affective Disorder. While Seasonal Affective Disorder (or SAD) is interesting in itself as a long-suffered from but only recently labeled illness, it is perhaps most interesting as an illustrative example of the complex interplay between neurological processes and the environment. The study of Seasonal Affective Disorder provides fascinating implications for how the brain, as a regulating feature of the nervous system, is clearly and strongly influenced by cues from the world outside. Defining Seasonal Affective Disorder is somewhat more complicated than it may seem at first glance. It is generally thought to be a recurring fall/winter depression experienced by susceptible people, those with a lower threshold for light-associated depression. SAD affects approximately 35 million Americans, 10 million with the disorder and 25 million with subsyndromal SAD (8). While most people who live in northern latitudes tend to experience some mild depression symptoms related to seasonal changes, up to five or ten percent of the general population may be affected by one or more of the diagnostic symptoms of Seasonal Affective Disorder (6). There is a clear link between latitude (probably proportional to the amount of exposure to daily direct sunlight) and the susceptibility to Seasonal Affective Disorder. For example, in Florida, less than 1% of the general population is thought to be affected by SAD, while in Alaska, the percentage of affected people may be as high as 10% (7).

The major diagnostic symptoms related to SAD are recurring periods of depression and mood change throughout several years of seasonal changes. Other symptoms include nighttime carbohydrate craving (sugars, starches and alcohol) and weight gain, irritability and anxiety, an inability to focus, guilt, lethargy, hopelessness, suicidal thoughts and decreased sexual appetite (2, 3, 5, 7, 8, 9). Many of these symptoms are easily confused with those of general depression or fatigue syndrome, making Seasonal Affective Disorder a difficult illness to diagnose . Unlike with general depression, however, people with SAD often have hypersomnia during the winter, oversleeping by as much as four hours. Often, a sense of drowsiness and fatigue continues throughout the day (9). Crucial in the diagnosis of SAD is the cyclical, seasonal pattern of depression (often lasting for four or five months) that comes as the days become shorter and goes as they become longer (9). Often with the longer daylight hours of spring, SAD sufferers become hyperactive or manic (6).

An example of a severe case of SAD involves a woman who felt a general sense of depression during the winters. Every year, she was able to deal with her symptoms on her own until the annual family vacation to Florida in February, when her children had their vacation from school. One year, however, the children's school vacation was delayed until March. No longer able to hang on, she attempted suicide (2). While this is an extreme case of the disorder, it clearly shows the direct link between season and mood. Interestingly, the effects of SAD are much stronger in women than in men. In fact, women are 4 to 5 times more likely to be Seasonal Affective Disorder sufferers than are men (11). Whether this is due to an increased propensity toward seasonal depression in women or to their increased likelihood of seeking medical assistance remains to be fully explained (14). SAD has also been noted in children (8).

What causes these severe seasonal mood changes? There are several possible explanations available for Seasonal Affective Disorder. The first relies on the more social and symbolic views of light and dark to explain darkness-related depression. According to this theory, lights and candles in winter celebrations have historically been used as an assault against the negative mood-producing effects of darkness. These celebrations have come to be associated with feelings of warmth and love. In reality, however, a dichotomy exists between how happy people think that they should be and how happy they truly are. The realization of this dichotomy, according to some, is cause enough for feelings of severe guilt and depression (2, 11). The mismatch between expected sensory input and actual sensory input causes a sense of discomfort, anxiety and depression. While this explanation may well help in understanding some of the social implications of depression around the holidays, it does not at all explain why people in the southern hemisphere suffer from Seasonal Affective Disorder during the short days of the southern winter ... in July (11).

Other, more plausible, explanations for seasonal mood changes correlate human circadian rhythms with those of other animals in how we deal with the decreasing winter photoperiod, the part of a 24-hour cycle that is light. While this theory may sound a touch extreme, depression associated with winter may be due to the forcing of the body to resist hibernation during the winter (8). While it may sound a touch extreme, human daily circadian rhythms are very similar to other animal natural seasonal cycles (hibernation, migration and estrous cycles) which require the adaptation to changing environmental cues in order to function properly (11). The brain uses "zeitgebers," or time givers, to synchronize the environmental factors that keep us in phase with their rhythms. The most obvious cue is light, but there are other cues that we use to keep us in rhythm (15). Keeping with this notion of adaptive circadian rhythms, we must first get a better understanding of the complex interplay between environmental cues (light/dark) and the functioning of the brain (chemical release, mood). Within this context, we can see how the same cues cause phase-delayed rhythms and mood problems (6).

The human internal "biochronological clock" has been set evolutionarily to be in general synchronicity with the 24 hour cycle of the rising and setting of the sun. In order to be awake and alert during the daylight hours for hunting and gathering, the internal rhythm has been set to sleep during the dark hours. The wake/sleep cycle, then, is largely in tune with the light/dark cycle. Where does this setting of the clock take place? The suprachiasmatic nucleus (SCN) just above the optic chiasm in the hypothalamus (where fiber tracts from the left and right retina cross over) seems to be the most likely location for the master clock (2, 8). Interestingly, it is here, around the visual center of the brain, that the connection is made between the internal setting mechanism and the sensory input from the outside world (2). Once the SCN receives input concerning environmental light, the information is sent via the sympathetic nervous system to the melatonin-producing pineal gland at the base of the brain. The melatonin is then sent out nightly into the blood stream. The amount of melatonin released by the pineal gland is determined by, and inversely proportional to, the quantity of light received. In other words, the more light received at the gland during the day, the lower the amount of melatonin released into the blood at night. During the shorter days and longer nights of winter, then, an increased amount of melatonin is released into the blood (The above from 2 and 14). Light, then, is a suppressor of melatonin release. This is another interesting example of the brain regulation chemical release and controlling behavior through inhibition rather than activation.

What does melatonin have to do with anything? We must now look at the effects of increased melatonin (controlled by environmental light/dark) on the wake/sleep cycle, the regulation of mood, and general well being. The normal sequence probably goes something like this. As the sun provides increased light in the morning, melatonin hormone (and serotonin neurotransmitter) release into the bloodstream is lowered. This causes body temperature to rise, triggering the body processes to move towards their wake state. Those affected by Seasonal Affective Disorder, however, may actually have a higher light set point needed to trigger the wake state processes (8). For these people, as the nights grow longer in the winter and melatonin levels remain high, environmental cues are not strong enough to trigger the switch from the sleep to the wake state and the person continues to sleep, or, once awake, to feel drowsy (2). This is probably not an abnormality of the internal clock itself but more of an inadequacy of the response system (either in chemical or, possibly, retinal sensitivity) to the environmental light cues during the winter (15).

Along with abnormal sleeping patterns, several other symptoms of seasonal depression can be explained by phase-delayed circadian rhythms. The carbohydrate craving symptomatic of many SAD sufferers fits well into this model. Because of the higher light set point for melatonin suppression, melatonin levels remain high and body temperature remains low. People with SAD, then, crave quick energy-providing carbohydrates. Perhaps like other brain processes that involve an expected sensory input that is different from the real input, the mismatch in state between what is expected in circadian daily light/dark pattern and the actual level of light perceived leads to feelings of unease and discomfort. While there is much evidence for the significant role of phase-delayed circadian rhythms in explaining seasonal mood changes, a definite correlation between melatonin secretion (and neurotransmitters like serotonin and dopamine release) and psychiatric disorders such as SAD has not yet been fully proved (15).

Other studies have used phase-delayed circadian rhythms to explain seasonal mood change, but using different environmental cues. According to one theory, it is the decrease in strength of environmental magnetic fields in the winter that elicits melatonin secretion. Circardian rhythms are then desynchronized, resulting in seasonal mood change. For this, magnetic therapy is the recommended treatment (15). If none of these theories grabs you, there are other areas under investigation. Studies have found correlations between SAD and premenstrual tension (9) and between SAD and a more general biological or hormonal imbalance (9). One study implies that there is a genetic component to the disorder. It was found that when one of a set of twins was afflicted with the disorder, about one half of their twins were also found to have SAD (11). Like so many expressions of brain function, it is largely impossible to pinpoint the exact cause of seasonal depression. It is obvious, however, that there is continuous exchange between environmental cues and the adjustment of circadian rhythms related to mood.

The most common form of treatment for people with Seasonal Affective Disorder is through the interaction of light with the eyes, known as bright light therapy (BLT) or phototherapy (6). This consists of supplying the person with bright, broad-spectrum light. Looking indirectly at the high-wavelength lights (fluorescent, incandescent or full-spectrum) for several hours a day will decrease the amount of melatonin in the blood. After several days or weeks of exposure to strong light, wake/sleep patterns will generally become adjusted to the normal winter phase (4). Artificial light serves to readjust the body's set point for light, restoring normal patterns of hormone secretion from the brain to the bloodstream and resetting the internal circadian clock to be in synch with environmental rhythms (12, 14). With these changes comes a lifting of the apathy, lethargy and depression associated with Seasonal Affective Disorder (2). The manipulation and readjustment of the circadian system takes some time. This is because the SCN is programmed to release melatonin according the schedule of the day before. In other words, yesterday's day length, from sunrise to sunset, will determine tonight's melatonin secretion (16). Other possible therapies for SAD include melatonin treatment, serotonin specific reuptake inhibitors and negative air ionizing therapy (12).

Seasonal Affective Disorder seems to be a largely non-life-threatening, easily treatable illness with hundreds of websites dedicated to it. Why examine it further? Before the 1980’s, SAD was not even considered a clinical illness or a psychiatric disorder. Only recently has mood alteration due to environmental changes been considered part of the medical domain. My own father might use to word "psychobabble" to describe it and suggest it be thrown on to the pile with the other 80s "nouveau-diseases." He might say that it is simply a prolonged poor mood state that requires some will power, some involvement of the I-function (although my father would probably not use that terminology) to control and overcome. There is some evidence that establishing a strong mindset will help to control some of the symptoms associated with SAD (2). There is definitely some give and take, some interaction between the control of mood by the brain through outside forces and through will. Can we assert, though, that aberrant set points and circadian rhythms can be changed through sheer will? No. If we subscribe to the notion that the environmental cues are adjusting our brain activity, our hormone release, and thus, our behavior, can we control how we react to it through mental obstinacy alone? No. Both seasonal patterns and our own wills can affect our internal clocks, but neither can determine its schedule or permanently change it.

So, while the more political, "what makes a disease a disease" question may seem off the topic, it does bring in the important question of the origin of illness and healing. Is it from within us or from outside? If healing comes from a pill, a shot, a box of light, does that make the illness real? Although my father would probably have steam coming out of his ears by this point in reading this paper, the lesson here is not in whether or not Seasonal Affective Disorder treatment should be covered by health insurance. Instead, it should be understood within the larger framework of how minds interact with the environment. Seasonal mood change is a clear and undeniable example of what happens when something goes wrong in the exchange of information from the outside to the inside. By examining seasonal mood change, the string linking the environment, circadian rhythms, the brain and behavior is a little tighter.


1. The Book of Genesis

2. Seasonal Affective Disorder (SAD): About light, depression and melatonin

3. National Alliance for the Mentally Ill Helpline Fact Sheet: Seasonal Affective Disorder (SAD)

4. Apollo Light Systems: Treatment Guidelines for Patients with Seasonal Affective Disorder

5. Seasonal Affective Disorder (aka Winter Depression)

6. Shedding Light on the Blues: Rx for Winter Depression

7. UBC/VHHSC Mood Disorders Clinic: Info about Seasonal Affective Disorder (SAD)

8. NIMH, JAMA Shed Light on Seasonal Affective DisorderJohnson, Roger and Somers, Sandra.

9. Severe Winter Blues Could Signal a Treatable Disorder

10. Hospital Happenings: Seasonal Affective Disorder

11. The CyberCouch Library Am I Blue or just S.A.D.? Pearson, Daniel.

12. Down in the Doldrums: Fall is SAD Season. Adler, Carlye.

13. The Really Helpful Program Factsheet

14. Winter Depression, Seasonal Affective Disorder (SAD) and Light Therapy.Sunbox Designs Ltd.

15. Melatonin: A Review.Goldman, Arlene.

16. Circadian Rhythms Factor in Rapid-Cycling Bipolar DisorderLiebenluft, Ellen.

17. Apollo Light Systems – Insurance Information

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This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.

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