Women with ovulatory cycles have a biphasic change in body temperature, reduced subjective sleep quality premenstrually and at menstruation, while sleep homeostatic mechanisms, as reflected by slow wave sleep (SWS), are unaltered. The mid-luteal phase increase in body temperature is associated with more stage 2 sleep, higher spindle frequency activity, reduced REM sleep, and elevated heart rates during sleep when compared to the mid-follicular phase. Based on a few studies the effects of oral contraceptives (OC) appear small: women taking OCs have more stage 2 compared to naturally cycling women but less SWS than naturally cycling women in the luteal phase.