Associations between sex hormones, sleep problems and depression 217 1. Introduction 1.1. Depression, sleeping problems and gender differences in prevalence Depression and sleep problems are both significant issues in today’s society; they have a high prevalence and impact daily functioning and quality of life strongly. Depression and sleep problems are often intertwined: insomnia symptoms, such as difficulty falling asleep and waking during the night, are common symptoms of depression (American Psychiatric Association, 2013; Fava, 2004; Hayashino et al., 2010) and poor sleep quality, insomnia and very long or short sleep duration could be risk factors for depressive episodes(Baglioni et al., 2011; Lustberg & Reynolds, 2000; Neckelmann et al., 2007; Paunio et al., 2015; Zhai et al., 2015). Differences in sleep physiology between depressed patients and controls have been described using electroencephalogram (EEG) measurements. Studies have found that depressed patients often have longer sleep onset latencies (SOL) (Lovato & Gradisar, 2014), have lower delta sleep ratios (meaning less deep sleep in the beginning of the night) and spend less time in slow wave sleep (SWS), arrive at REM (Rapid Eye Movement) sleep faster and experience more REM sleep (Pillai et al., 2011). Interestingly, these altered sleep markers are also seen in healthy controls who are at high risk for depression (Pillai et al., 2011), which suggests that these sleep markers may predict future depressive episodes (Palagini et al., 2013). After remission of depression, the differences in REM sleep, slow wave sleep and delta sleep ratios tend to reduce (Wichniak et al., 2013). There is a widely known sex difference in the risk for sleep problems and depression: it is found that women are 1.5 times more likely than men to suffer from insomnia (Klink et al., 1992; R. H. Y. Li et al., 2002; Zhang & Wing, 2006) and two times more likely to suffer from depression (Kessler, 2003; Kuehner, 2003), even after adjustment for race, income and employment (Rai et al., 2013). In contrast, sleep EEG studies show that women on average have more SWS than men (Santhi et al., 2016) and higher power in theta and delta frequencies (Carrier et al., 2001), indicating they actually experience deeper sleep than men.
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