Seasonal Affective Disorder (SAD)

By Dr. Gregg D. Jacobs
September 27, 2004

Why is it that, on sunny days, our mood is more upbeat than on dull, gray days? Why do we experience the irresistible urge to get away to a sunny climate in the dead of the winter? The answer is that light energizes us and improves health. In fact, for health problems like depression, light can even be more effective than drugs.

Consider how, for almost our entire evolution as hunter- gatherers, we were exposed to the natural cycles of sunlight during the day and darkness at night. But with the advent of modern technology, we have significantly altered our exposure to light and darkness. Electricity has allowed us to stay awake and active well into the night while we have moved our daily activities from outdoors to indoors.

For these reasons, we get little exposure to natural sunlight or true darkness. We spend our days working or living indoors away from natural light while, at night, we are exposed to indoor light and the constant illumination of city-lit skies. How much time do you spend each day in bright sunlight? If you are like most, probably very little. Studies have shown that, no matter where people live, they obtain only one hour of sunlight on average during the day. And when was the last time you saw the luminous Milky Way or a brilliance of the stars, something that our ancestors saw regularly? The "light pollution" of urban environments means that many people also don't receive exposure to true darkness anymore.

The main reason that we obtain so little sunlight is that most of us work indoors. A brightly lit room has about 500 luxes of light (a lux is the equivalent of the light from one candle) compared to 10,000 luxes at sunrise and 100,000 at noon on a summer day. To the brain, spending the day indoors is equivalent to spending the day in darkness. The problem is compounded in the late fall and winter months, when days grow shorter and many of us curtail outdoor activities. Our distant ancestors evolved on the Savannas of Africa but many of us now live in northern latitudes where ambient sunlight is significantly diminished in the winter.

That lack of exposure to bright light can adversely affect our daytime mood, energy, and alertness level has been recognized since antiquity. The initial scientific studies on the effects of light on mood involved depression. In the early 1980s, scientists at the National Institutes of Mental Health found that a patient who experienced depression annually during the fall and winter months (what is called 'seasonal depression') improved from daily treatment with bright light exposure. That single case study has since led to defining seasonal affective disorder, or SAD (a disorder which is characterized by depression and sleep disturbance in the winter months), and extensive testing of the efficacy of bright light therapy.

There is now a scientific consensus that bright light therapy is effective for SAD, a disorder which may overlap with chronic and intermittent depression. Because those living in northern climates spend almost six months a year in winter-type lighting conditions, SAD symptoms may be present a majority of the time. That is why people in northern latitudes are more prone to seasonal affective disorder.

The seasonal changes in mood observed in SAD patients may represent the extreme end of a behavioral spectrum, which can also be observed in normal, non-depressed individuals to a lesser extent. For the general population, studies show that mood and energy are poorest during the winter months, which have the shortest days and therefore the least amount of sunlight. A population survey conducted in New York City indicated widespread occurrence of seasonal mood swings and other symptoms. The people who were surveyed rated the degree of seasonal change in sleep length, social activity, mood, weight, appetite, and energy by indicating when they felt best and worst, socialized the most and least, and so on; they also rated the degree to which seasonal changes presented a personal problem in their lives. Half the respondents reported lowered energy in the late fall through the winter, and a quarter of the respondents reported that poorer mood in the late fall and winter posed a personal problem. When generalizing these findings to the millions of people living in New York City, you can appreciate the number of people who experience SAD-type symptoms. Although the symptoms are most prevalent in the fall and winter months, they may be experienced to a lesser degree in the spring and summer if little time is spent outdoors.

Some scientists believe that the lack of exposure to sunlight affects the population as a whole and may be contributing to an overall increase in mood disorders in the general population; and, that widespread exposure to supplementary light, at least in the winter, might have widespread benefit, particularly for those individuals who are "normal" but are more vulnerable to seasonal depressive symptoms.

Despite the therapeutic effects of light on mood and sleep, surprisingly little is known about the underlying mechanisms through which bright light exerts its effects. Presumably, the antidepressant and mood-energizing effect of light on mood is due in part to the neurobiological effects of light on an area of the brain called the suprachiasmatic nucleus, which is located in the hypothalamus of the brain. This nucleus contains a circadian pacemaker that acts as a "clock for all seasons" by being tuned to seasonal changes in light. For mammals, changes in light and darkness play an important role as a chemical mediator of the effects of season on breeding and other behaviors such as eating, sleeping, and weight. In many animals, including humans, the circadian pacemaker is able to take seasonal variations in the duration of darkness and light into account by detecting seasonal changes in day length and making corresponding changes in the brain.

Although we have retained these intrinsic seasonal responses to changes in light and dark, the responses are now masked by indoor environments, control over lighting, shift work, and air travel. Instead of being exposed to purely natural light, we expose ourselves to artificial indoor light and virtually eliminating exposure to true darkness as a consequence of nighttime lighting. This disruption in the effect of light on the circadian pacemaker is believed to be responsible for the increased predisposition to seasonal changes in mood in vulnerable individuals, possibly through disruptions in the balance of neurotransmitters like serotonin that are involved in mood.

Besides affecting our mood, light affects our sleep, which is important for restoring energy. Sleep and body temperature are directly influenced by the effect of the daily cycles of light and darkness on melatonin, a naturally occurring hormone found in the brain. When sunlight enters the eyes, melatonin levels decrease, which signals body temperature to rise and promotes wakefulness. Darkness causes melatonin levels to rise and body temperature to fall, which promotes sleep.

By reducing our exposure to bright natural light and true darkness, melatonin secretion and the body temperature rhythm are altered, which can exacerbate sleep difficulties. This explains why up to 90% of the blind experience sleep problems.

You can minimize SAD and your sleep by increasing your exposure to sunlight. Aside from the obvious ways of increasing your exposure to natural bright light, you may want to consider the use of artificial bright light boxes. These devices contain special bulbs that emit 5000-10,000 luxes of light, which is equivalent to a sunrise or sunset. They are used for about 30 minutes while reading or watching television to increase early or late day exposure to bright light. Several studies have demonstrated that using bright light boxes in the evening can delay the body temperature rhythm and effectively minimize early morning insomnia and many studies have demonstrated that the boxes are effective for SAD. Light boxes can be rented from medical supply companies or purchased from an increasing number of manufacturers. Some insurance companies even reimburse the cost of light boxes if they are prescribed by a physician for SAD or insomnia.