SAD

Most animals experience dramatic seasonal cycles: they migrate, hibernate, mate, and molt at specific times of the year. These cycles appear to be hardwired; they occur even when the temperature is held constant and the light and dark periods are varied. But humans are among the least seasonally sensitive creatures, having only a vestige of seasonal effects known as seasonal affective disorder (SAD), an extremely mild version of the cyclical responses animals experience. Only about 5 percent of adults overtly sense the seasonal changes and suffer from SAD during the winter days of longer darkness. Amazingly, light therapy looking into a light that mimics sunlight or merely sleeping until dawn helps the people with SAD in northern latitudes. Would these therapies be effective on people living at the Equator?

Answer

Yes, if they suffer from SAD. At first one might think that the variation in the length of day and night changes so little from January to June that no one living at the Equator would suffer from SAD. This reasoning is true if everyone went to bed at sunset and arose at sunrise. The increasing light at sunrise would trigger the start of another circadian rhythm that brings about biochemical changes in our bodies.

But even people living near the Equator are tuned no longer into the rise and setting of the Sun. There are problems in their circadian rhythms because the bright artificial lighting means that people stay awake long past sunset, delaying and shifting the maximum in certain biochemical cycles beyond their evolutionary time of day. In particular, greenish light from televisions and clock radios passes into the eyes, even through closed eyelids while asleep, to trigger the pineal gland to initiate some of the biochemical circadian rhythm shifts.