Diurnal and circadian variation of sleep and alertness in men vs. naturally cycling women.
Level 3 - non-randomized controlled study
Non-randomized controlled laboratory study
PubMed 27621470 · doi:10.1073/pnas.1524484113
What was done
Researchers compared 11 naturally cycling women (8 evaluated during two menstrual cycle phases and 3 during the midfollicular phase only) with 15 men. Participants completed an ultradian sleep-wake procedure consisting of 36 cycles alternating 60-minute wake periods with 60-minute nap opportunities. Core body temperature, salivary melatonin, subjective alertness, and polysomnographically recorded sleep were measured.
What was found
The abstract reports directional findings without numerical values, confidence intervals, or effect sizes. All analyzed variables showed significant diurnal and circadian variation. Compared with men, women showed a significant phase advance in core body temperature rhythms (with no phase shift in melatonin rhythms) and phase advances in sleep measures and subjective alertness. Women also had a higher amplitude of diurnal and circadian variation in alertness, driven by a larger drop at the nocturnal nadir.
Why it matters
These findings suggest that circadian phase alignment differs between men and women, providing a physiological explanation for sex differences in sleep disturbance vulnerability and night shift work adaptation.
Limits
The sample size is very small, with only 26 total participants, and 3 women completed only one testing phase. The 60-minute alternating wake/nap schedule is artificial and does not reflect real-world sleep patterns. Numerical metrics are absent from the abstract.
Cited by
- context Human core body temperature must increase by approximately 1 to 3 degrees in order to wake up feeling refreshed and energized.