An ovulatory menstrual cycle is characterized by fluctuating levels of progesterone. Progesterone, a gonadal hormone known for its soporific and thermogenic effects, is present in negligible levels prior to ovulation and in high levels after ovulation. To describe and compare sleep patterns in relation to ovulatory cycles and premenstrual mood state, sleep was monitored in healthy women at two phases of the menstrual cycle. Results indicated that rapid-eye-movement (REM) latency was significantly shorter during the postovulatory (luteal) phase compared to the preovulatory (follicular) phase, but there was no significant difference in latency to sleep onset or the percentage of REM sleep. While there were no menstrual cycle phase differences in the percentages of various sleep stages, the women with negative affect symptoms during the premenstruum demonstrated significantly less delta sleep during both menstrual cycle phases in comparison with the asymptomatic subjects.