Sex difference in the near-24-hour intrinsic period of the human circadian timing system

TLDR

Women’s circadian rhythms of melatonin and body temperature are set earlier than men’s, with women also waking earlier and preferring morning activities, yet the neurobiological basis for this alignment difference remains unknown despite animal evidence of sex differences in circadian period. The study aimed to determine whether a sex difference in intrinsic circadian period underlies the observed circadian alignment disparity between men and women. Researchers measured intrinsic circadian period in 157 adults (52 women, 105 men) during a month‑long inpatient protocol designed to eliminate confounding factors. The mean intrinsic period was 24.15 h, with women’s period significantly shorter (24.09 h) than men’s (24.19 h), and a higher proportion of women (35 %) had periods under 24 h versus 14 % of men, suggesting implications for sex differences in sleep duration and insomnia.

Abstract

The circadian rhythms of melatonin and body temperature are set to an earlier hour in women than in men, even when the women and men maintain nearly identical and consistent bedtimes and wake times. Moreover, women tend to wake up earlier than men and exhibit a greater preference for morning activities than men. Although the neurobiological mechanism underlying this sex difference in circadian alignment is unknown, multiple studies in nonhuman animals have demonstrated a sex difference in circadian period that could account for such a difference in circadian alignment between women and men. Whether a sex difference in intrinsic circadian period in humans underlies the difference in circadian alignment between men and women is unknown. We analyzed precise estimates of intrinsic circadian period collected from 157 individuals (52 women, 105 men; aged 18–74 y) studied in a month-long inpatient protocol designed to minimize confounding influences on circadian period estimation. Overall, the average intrinsic period of the melatonin and temperature rhythms in this population was very close to 24 h [24.15 ± 0.2 h (24 h 9 min ± 12 min)]. We further found that the intrinsic circadian period was significantly shorter in women [24.09 ± 0.2 h (24 h 5 min ± 12 min)] than in men [24.19 ± 0.2 h (24 h 11 min ± 12 min); P < 0.01] and that a significantly greater proportion of women have intrinsic circadian periods shorter than 24.0 h (35% vs. 14%; P < 0.01). The shorter average intrinsic circadian period observed in women may have implications for understanding sex differences in habitual sleep duration and insomnia prevalence.