While a number of studies demonstrate that gonadal steroids regulate circadian rhythms in animals, the issue has received little attention in humans. The question is relevant to our understanding of gender differences in the phenomenology of depression, and of changes in the sleep-activity cycle that are seen in affective illness and during the menopause. In this paper, the literature demonstrating that gonadal steroids regulate circadian rhythms in animals is reviewed, along with the limited human literature. A hypothesis is forwarded suggesting that, in humans, estrogen shortens the circadian period, lengthens the sleep phase, advances sleep onset, and consolidates sleep. A study in progress is described which is designed to test the effects of the hypogonadal state, estrogen, and progesterone on circadian rhythms in women.