Female reproductive steroids, oestrogen and progesterone, not only affect reproductive function, but also thermoregulation and sleep. Chronic administration of synthetic steroids, as occurs in women taking oral contraceptives, may affect these regulatory systems differently from endogenous oestrogen and progesterone. We therefore investigated body temperature and sleep in ten young women taking oral contraceptives, in the active and placebo phases of the contraceptive pack, and compared them to a group of nine women with ovulatory cycles, in the mid-follicular and mid-luteal phases. Body temperature was raised throughout 24 h in the women taking oral contraceptives in the active phase, and in the naturally cycling women in the luteal phase, compared to the follicular phase. The women taking oral contraceptives in the placebo phase, however, continued to have raised body temperatures, similar to those in the active phase, indicating a prolonged action of synthetic reproductive steroids on body temperature. Sleep also was influenced by the endogenous and synthetic reproductive steroids, but independently of body temperature. The women taking oral contraceptives had more stage-2 non-rapid eye movement sleep in the active phase, both compared to their placebo phase and the naturally cycling women. The naturally cycling women, however, had more slow wave sleep in the luteal phase compared to the contraceptive group of women. Exogenous reproductive steroids therefore influence body temperature and sleep differently from endogenous progesterone and oestrogen.