This chapter reviews what is known about menstrual-related hypersomnia, a rare disorder of recurrent hypersomnia that is temporally linked with menses. In light of the rareness of cases of menstrual hypersomnia, the chapter also discusses the influence of the menstrual cycle on levels of daytime sleepiness in women during normal cycles and in those with premenstrual syndrome. Evidence indicates that ovulation is necessary for hypersomnia episodes to appear in affected individuals. Hypersomnia episodes were associated with ovulatory but not anovulatory cycles, as monitored with basal temperature measurements, in one patient, and prevention of ovulation with oral contraceptive treatment effectively alleviates the hypersomnia. Women experience increased feelings of sleepiness and fatigue around the time of menstruation. Laboratory findings show that women with premenstrual syndrome (PMS) are able to maintain wakefulness under soporific conditions despite their subjective sleepiness.