Researchers suspect that seasonal affective disorder, first reported in 1984, has something to do with circadian rhythms thrown off by short, dark days. At first, Vox reports, scientists connected SAD to excessive production of melatonin; now they think it has more to do with the mismatch of melatonin production and sleep schedules.

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Either way, short periods sitting under a special lamp is recommended as a treatment, and researchers have wondered whether the the effects of phototherapy might be able to treat chronic mood disorders. Now, Nautilus reports, "research into the circadian underpinnings of chronic depression, bipolar disorder, Alzheimer’s disease, and fatigue suggests that light could help these patients readjust too."

Phototherapy has long been used to treat certain conditions: the power of artificial sunlight for skin disorders was demonstrated over a century ago. The doctor who won the 1903 Nobel Prize in medicine found that an hour a day of light therapy could help cure smallpox, and lupus vulgaris, a form of tuberculosis. But it's only in the past couple of decades that researchers have looked at light treatment as a possibility for people suffering year-round from depression or other diseases.

In a 1992 study, two dozen veterans exposed to a bright light treatment saw a decline in depression and bipolar symptoms compared to a control group, exposed to a dim, red, light. A few more recent studies have since shown that there are also positive anti-depressive effects of light therapy for pregnant women and elderly people, Nautilus reports.

This suggests that light therapy could at least augment other forms of treatment for several types of depression. Last year, a study suggested the treatment could work for anxiety, too. These studies are small. But while skin therapies use ultraviolet light, SAD lamps use a smaller, safer spectrum. The side effects of sitting under these sunlamps are almost nonexistent, and even a possibility of a benefit could make the treatment worthwhile.