Seasonal rhythms characterize the behavior and physiology of many animals, including humans. For some unfortunate people, winter brings a low period that may become a profound depression. Sometimes the winter depression alternates with summertime mania (Blehar and Rosenthal, 1989). In wintertime, affected people feel depressed, slow down, generally sleep a lot, and overeat. Come summer, they are elated, energetic, and active, and they become thinner. This syndrome—called seasonal affective disorder (SAD)—appears predominantly in women and generally starts in early adulthood.

Some early reports suggested a positive correlation between latitude and the frequency of SAD: the farther from the equator, the more cases of SAD. But a study in a country at a far northern latitude—Iceland—where a relatively high rate of SAD would be expected, failed to confirm this relationship (Magnusson and Stefansson, 1993); and in general, a relationship between latitude and SAD has not been very evident (Mersch et al., 1999). Nevertheless, seasonal rhythms are controlled by the length of the day, so researchers asked whether seasonal changes in exposure to sunlight might cause SAD.

To examine that prospect, some investigators have treated SAD sufferers with doses of bright light, to see whether it acts as an antidepressant. The effectiveness of light therapy, or phototherapy, in SAD is fairly well established (Golden et al., 2005), and in many ways light therapy resembles treatment with traditional antidepressant drugs (Moscovitch et al., 2004); there is even a dose-response relationship for light therapy. Light therapy may be most effective when administered immediately upon awakening in the morning (Lewy et al., 1998). A person receiving phototherapy is shown in Figure 1.

Figure 1  Light therapy for SAD
(Photo courtesy of Uplift Technologies.)

One important biological effect of light is that it suppresses melatonin, a hormone that is normally released from the pineal gland at night. Hypothesizing that SAD may result from a misalignment of melatonin secretion with sleep, one group administered melatonin as a treatment. Most patients benefited only if the melatonin was taken in the afternoon (Lewy et al., 2007).


Blehar, M. C., and Rosenthal, N. E. (1989). Seasonal affective disorders and phototherapy. Report of a National Institute of Mental Health-sponsored workshop. Archives of General Psychiatry 46: 469–474.

Golden, R. N., Gaynes, B. N., Ekstrom, R. D., Hamer, R. M., et al. (2005). The efficacy of light therapy in the treatment of mood disorders: A review and meta-analysis of the evidence. American Journal of Psychiatry 162: 656–662.

Lewy, A. J., Bauer, V. K., Cutler, N. L., Sack, R. L., et al. (1998). Morning vs evening light treatment of patients with winter depression. Archives of General Psychiatry 55: 890–896.

Lewy, A. J., Rough, J. N., Songer, J. B., Mishra, N., et al. (2007). The phase shift hypothesis for the circadian component of winter depression. Dialogues in Clinical Neuroscience 9: 291–300.

Magnusson, A., and Stefansson, J. G. (1993). Prevalence of seasonal affective disorder in Iceland. Archives of General Psychiatry 50: 941–946.

Mersch, P. P., Middendorp, H. M., Bouhuys, A. L., Beersma, D. G., et al. (1999). Seasonal affective disorder and latitude: A review of the literature. Journal of Affective Disorders 53: 35–48.

Moscovitch, A., Blashko, C. A., Eagles, J. M., Darcourt, G., et al. (2004). A placebo-controlled study of sertraline in the treatment of outpatients with seasonal affective disorder. Psychopharmacology (Berlin) 171: 390–397.