According to J. Raymond DePaulo Jr., M.D., professor of psychiatry at Johns Hopkins University School of Medicine:
Although you might think that more people would tend to commit suicide in the dark days of winter, in fact, the peak seasons for suicides happen to be early spring and early autumn. In the United States, suicides for both men and women peak in April and May, while suicides in women alone show a rise in September and October. Why should there be such seasonal variation?
Seasonal change in light cause many behavioral and physiological changes, including precipitating depression. These changes have an impact on the levels of neurotransmitters like serotonin. They affect the sleep and temperature regulators of the body, the action of the endocrine and hormonal systems, and the production of testosterone and estrogen. Changes in these systems can play havoc with mood, particularly in a person with affective illness.