The present article reviews direct and indirect evidence of the effects of sex steroids on different aspects of sleep. It begins with a review of what is known about the effects of steroid hormones on sleep and on central nervous system processes related to sleep, such as the GABA-ergic system, in animals. It continues with a review of the effects of exogenous hormones on human sleep and a review of studies comparing sleep during hypogonadal states secondary to surgical or natural menopause. The article proceeds to review the data on the effects of the menstrual cycle on both subjective and objective aspects of sleep and circadian temperature and melatonin rhythms in samples of healthy women, women with premenstrual dysphoric disorder, and women with primary insomnia. Then, the article reviews gender differences in sleep during depression and raises the possibility that sex steroids moderate these differences. Finally, the article concludes with a discussion of the implications of the data reviewed for basic clinical, and methodological aspects of sleep research.