Editors' ChoiceCircadian Rhythms

Sex and the night shift

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Science Translational Medicine  11 May 2016:
Vol. 8, Issue 338, pp. 338ec75
DOI: 10.1126/scitranslmed.aaf9188

Sex differences in sleep and circadian rhythms have been reported in both animals and humans. Observed sleep and circadian differences between males and females include the duration of slow wave sleep, endocrine modulation of circadian control, and the timing of clock gene expression rhythms in the brain. Despite this, women are underrepresented in sleep and circadian research leading to a gap in our understanding of why certain sleep and circadian disturbances have greater effects in women compared with men. In a recent study, Santhi et al. examined circadian and sleep-wake regulation of cognitive function in men and women under conditions mimicking jet lag or shift work.

Sixteen men and 18 women underwent forced desynchrony to a 28-hour sleep/wake cycle for 10 days. Participants had multiple sleep and circadian parameters monitored throughout the study, including the concentration of melatonin in plasma, total sleep time, and slow wave activity. Cognitive assessments for attention, memory, and other functions were also performed every 3 hours. Although reported levels of sleepiness during the night did not differ between women and men, women were found to have greater impairment in thinking and memory compared with men after a long time awake combined with an adverse circadian phase (mimicking the conditions of jet lag or shift work). A key finding of the study was that accuracy on a cognitive test was reduced to a greater degree in women than in men at a time that would coincide with the end of the night shift (early morning hours).

Prior studies have shown that the menstrual cycle has a circadian effect on cognition. A major limitation of the current study was the inability to control for different phases of the menstrual cycle. Despite this limitation, the study highlights the importance of including women in more sleep and circadian research studies. It also emphasizes the importance of the National Institutes of Health’s initiative to include sex as a biological variable in animal and human studies. Circadian rhythm disturbance is adversely associated with a broad range of human diseases, including cardiovascular and neurodegenerative disorders. Future studies of sleep and circadian biology need to take into account potential sex differences to fully elucidate potential adverse effects of sleep disturbances on health.

N. Santhi et al., Sex differences in the circadian regulation of sleep and waking cognition in humans. Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1521637113 (2016). [Full Text]

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