Seasonal Depression and the Circadian Cycle
By Diane Grenier FRIENDLY LINK

Come winter, we all dream of spending time "in the sun" because we feel a need to counter the fatigue and the blues we experience during the shorter winter days. However, for some people these seasonal blues are really a depressive state known as seasonal affective disorder or SAD. In the United States one person in twenty experiences depressive symptoms such as apathy, trouble concentrating, weight gain and fatigue, often accompanied by physical complaints, between October and March. It is ten times more common in the northern part of the country than in the south.

It seems the circadian cycle (length of days and nights) which varies according to the seasons has a direct influence on the rhythms of our physiological processes. When there is a lack of synchronization of daylight cycles and daily social and cultural activities it can lead to SAD. The problem can be effectively treated in seventy percent of cases with light therapy (using a special lamp) which increases exposure to light during the day. According to some researchers, SAD could be reduced if we were more attuned to the natural rhythms of sunrise and sunset, and adjusted the time we spend awake and sleeping accordingly.

Specialised cells in the retina of the eye transmit information about daylight to the hypothalamus nerve centers in the brain. These drive the daily fluctuations in blood pressure, stress hormone cortisol and body temperature. They also act on the brain's pineal gland which secretes -- only at night -- melatonin to promote sleep. These nerve centers have long been believed to regulate our internal clock. However, recent research has shown that there is more to it. Genes are also involved at the cellular level in the daily regulation of organ activity. With their own biochemical clocks, organs can function independently of the brain clock. Then various biological activities in the body are regulated by a number of rhythmic mechanisms whose combined motions promotes harmony and balance.

This synchronization can be disturbed at times without consequence, i.e. we can eat late and work all night. However, even if some functions can change their timetable, others are very dependent on the circadian cycle. We can, for example, vary meal and sleep times as we wish but we cannot decide of the level of melatonin or cortisol nor our body temperature at any given time. It is as if some parts of the body stay awake while at the same time others sleep. When our daily habits are perturbed, it may take days or even weeks to return to the normal synchronization of all internal clocks. This would explain the problems caused by jet lag and also why those who work evening or night shifts have an increased incidence of heart disease, gastrointestinal complaints and, of course, sleep problems. Night owls and party animals take note. If you don't keep all your clocks in sync, it taxes your body! Even slight deviations in the light-dark cycle have incidences, and some people can be affected by fatigue, mood swings or problems like SAD in winter. During those months, longer nights increase melatonin secretion even though our daily life remains essentially the same and forces us to function without daylight for several hours.

The circadian cycle and the seasons are very important to some animal species which adjust by going through hibernation, migration and mating periods. Human beings on the other hand are among the least seasonally sensitive creatures around. We can escape the drastic habits of animals because humans evolved in the tropics. There, the seasons vary little so that their impact on biological cycles is generally trivial, however we can't escape it all and some influences remain.

People who vacation down south during the winter months feel revived because the nature-culture gap of winter is lessened. For those of us who cannot travel to the sun in the winter, a good lifestyle can prevent the winter blues. Spending time outdoors in the daylight, going to bed at a reasonable hour and sleeping enough are recommended for good physical and mental health. For those who suffer from mental illness, these habits are essential in order to properly manage a disease which often leads to changes where day and night patterns can be reversed.

(See Times of Our Lives by Karen Wright, Scientific American, Sept. 2002, or  www.sciam.com ;  for info on SAD check: University of British Columbia: www.psychiatry.ubc.ca/mood/sad ; Direct SAD: www.fhs.mcmaster.ca/direct/subpages/pu_depcons.html ; The Society for Light Treatment and Biological rhythms: www.slbr.org )