Sex steroid secretions are generally synchronous with the circadian rhythm and sleep, and there is evidence that prolactin secretion is sleep-dependent. Polysomnographically assessed changes in sleep during the menstrual cycle are characterized by increased EEG activity in the 14-15-Hz (sleep spindle) range in the luteal phase accompanying an increase in core temperature. There are no other consistent changes in sleep architecture associated with the menstrual cycle. The hot sweats which disturb sleep in menopausal women are attributable to oestrogen deficiency and are reduced by oestrogen replacement therapy. Although it is often assumed that the psychological changes during the menopause are attributable to chronic sleep disturbance caused by hot sweats, the evidence for this is uncertain. Sex steroids have also been shown to have a role in the aetiology of obstructive sleep apnoea and its treatment. It is clear that the sex steroids are all implicated in sleep and thermoregulatory processes, although we cannot as yet define their precise roles.