Sex hormones, insomnia, and sleep quality: subjective sleep in the first year of hormone use 135 1. Introduction Many people experience poor sleep quality: they report trouble falling asleep, staying asleep, or waking up early. Globally, between 36% and 57% of people report poor sleep (Ammar et al., 2021; Fatima et al., 2016; Hinz et al., 2017), although estimates vary widely. If the burden of these symptoms significantly impairs daily life it can be defined as insomnia disorder. Insomnia is the second most common mental disorder in Europe (Wittchen et al., 2011), with an estimated 30% to 36% of the population reporting at least one insomnia symptom (Morin & Jarrin, 2022). The burden of insomnia is not distributed equally over the population. In cisgender populations, women of reproductive age are 1.5 times more likely to experience insomnia (Zeng et al., 2020) and they report worse sleep quality than men, as seen in longer sleep onset latencies, more sleep disturbances, and lower sleep efficiency (Lee et al., 2020; Li et al., 2019). Although studies suggest that gender-specific factors, such as social expectations, care duties and a higher prevalence of mood disorders, contribute to more insomnia and worse sleep quality in women (Hislop & Arber, 2003; Kessler et al., 1993; Venn et al., 2008), epidemiological studies also suggest that biological factors, such as sex hormones, can also affect sleep quality. There are several indications of associations between sex hormones and sleep quality throughout the lifespan. Firstly, the sex difference in insomnia prevalence starts from puberty onwards, with cisgender girls reporting more insomnia than cisgender boys (Zhang et al., 2016). In this period the timing of onset of the first menstruation, which is triggered by sex hormone changes, is specifically associated with an increase in insomnia risk (Johnson et al., 2006). Secondly, symptoms of insomnia also increase during pregnancy (Hashmi et al., 2016) and perimenopause (Ciano et al., 2017), which are both life phases in which sex hormones strongly fluctuate. Lastly, in cisgender men, who experience less marked sex hormone fluctuations in their lifetime after puberty, there seems to be a bidirectional relationship between testosterone and sleep. Disturbed sleep seems to result in lower testosterone levels (Leproult & Van Cauter, 2011) and vice versa, lower testosterone seems to be associated with poorer sleep quality (Ruge et al., 2019) and with shorter sleep duration (Auyeung et al., 2015).
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