Seasonal affective disorder (SAD) is a type of winter-time depression experienced by people those who live in northern latitudes such as those of New York, Seattle, all of Canada, and Northern Europe. I believe it is primarily a disorder of sunlight/vitamin D deficiency.
Vitamin D, when administered in late winter, produces a positive effect on mood in only five days. One theory for this is that vitamin D stimulates the brain to produce more serotonin. In a wintertime experiment, serum vitamin D levels doubled in six months through supplementation and dramatically increased scores on a wellbeing assessment. Two groups were given either 1,000 IU or 4,000 IU of vitamin D daily. And although both groups improved, the higher dose produced better results.
In another investigation, researchers studied the association between vitamin D levels and the risk of mood disorders in the elderly. The results were impressive. Those whose vitamin D levels were deficient—defined as less than 20 ng/ml—had 11.7 times the incidence of depression when compared to those whose vitamin D levels were highest. Usually an association is considered meaningful when a measured factor correlates to a 50% increase or decrease. In this case, the correlation between vitamin D deficiency and risk of mood disorders was a staggering 1,169 percent! In addition, the researchers measured cognitive ability (mental capabilities). In two of four tests, those with vitamin D deficiency exhibited cognitive performances that were 5.22 times and 3.22 times poorer than those who were not deficient.
Everyone that has ever ventured outside on a sunny spring day after a long winter, or even visited a tanning bed during winter, know how much the mood is elevated by the experience. It is likely that the combination of light and vitamin D produce the effect. Keep your vitamin D levels high this winter!
 Lansdowne, A. et al. Vitamin D3 enhances mood in healthy subjects during winter. Psychopharmacology (Berl) 1998;135:319-23.
 Vieth, R. et al. Randomized comparison of the effects of the vitamin D3 adequate intake versus 100 mcg (4,000 IU) per day on biochemical responses and the wellbeing of patients. Nutr J 2004;3:8.
 Wilkins C. et al. Vitamin D Deficiency Is Associated With Low Mood and Worse Cognitive Performance in Older Adults. Am J Geriatr Psychiatry;2006;14:1032–1040).