Shifted sleep-wake cycles might differently influence brain function in men and women, according to a study.

Circadian rhythms affect brain function via the sleep-wake cycle, but whether the effects differ between men and women is unknown. Derk-Jan Dijk and colleagues compared the effects of circadian phase and sleep-wake cycles on the performance of men and women in several cognitive tasks.

During a 10-day sleep protocol, the sleep-wake cycles of 16 men and 18 women were rescheduled to a 28-hour day. With no external daylight or time cues, the brain's 24-hour clock desynchronized from the sleep-wake schedule, similar to the effect observed during shiftwork. Every 3 hours when participants were awake, the authors administered subjective assessments, such as reported sleepiness, and objective cognitive performance tests, including attention and motor control.

In both men and women, the effects of circadian phase and awake time were generally stronger for subjective assessments than for performance in objective tests. The circadian effect on cognition, however, was larger in women than in men, such that women were more impaired during the early morning hours for certain tasks.

While many factors affect sleep and cognition, the findings provide insight into factors that may contribute to cognitive differences, and future studies should use tests designed to account for any existing baseline differences in task performance between the sexes, according to the authors.

ARTICLE #15-21637: "Sex differences in the circadian regulation of sleep and waking cognition in humans," by Nayantara Santhi et al. To be published in PNAS the week of April 18.