Numerous Clinical Studies and Sleep Surveys Have Shown Pronounced Sex Differences in the Occurrence of Insomnia and Sleep

NUMEROUS CLINICAL STUDIES AND SLEEP SURVEYS HAVE SHOWN PRONOUNCED SEX DIFFERENCES IN THE OCCURRENCE OF INSOMNIA AND SLEEP pathologies.1-3 Although baseline sleep patterns appear to be fairly similar when healthy men and women are compared under strictly controlled laboratory conditions,4,5 in a real-life setting with all its environmental influences and its complex social context, sleep patterns and sleep quality may differ considerably between the sexes.6,7 Furthermore, sex differences in sleep may gradually develop and become rather prominent in the course of aging and with certain pathologies.2 Particularly, aging-related sleep changes and pathologies such as sleep apnea and delayed sleep phase syndrome are more common in men, whereas insomnia and sleep changes associated with affective disorders are more common in women. More generally, it has been suggested that sex differences in sleep, while subtle under baseline conditions, may increase in magnitude under biological or environmental challenges.1,8 Importantly, in our modern around-the-clock society, many people, both men and women, are living under increasing pressure of full agendas and psychosocial stress.9,10 To study sleep under challenging conditions, animal models are a useful approach because they allow for controlled experimental manipulations that would be difficult to perform in humans, particularly in the case of more severe challenges such as stress. However, so far, females have rarely been included in animal research, and, thus, our current understanding of sleep and how it is affected by environmental factors remains biased. In this context, the aim of the present study was to examine sex differences in sleep under both baseline and challenged conditions. We performed our study in mice because this species has become an important and frequently used model in sleep research.1113 We chose C57BL/6J mice, one of the most commonly used strains, because baseline sleep in the females of this strain varies little over the estrous cycle.14 Therefore, a sex comparison would not be complicated by cyclic variations in female sleep. We first measured sleep in young adult male and female C57BL/6J mice under baseline conditions. Then, to study sex differences in sleep under challenged conditions, mice were subjected to 6 hours of sleep deprivation or 1 hour of restraint stress, both of which were applied at the beginning of the light phase, the main resting phase in mice.12,13 Sleep deprivation was used to challenge the normal sleep homeostatic regulatory mechanisms12 and restraint stress was used as a more severe challenge that is known to affect sleep in male mice.15