Chapter 7 198 misalignment and a chronic lack of sleep (Baron & Reid, 2014; Caliandro et al., 2021; Rutters et al., 2014). The variability in human chronotype is mostly determined by genetic variation in clock genes (Archer et al., 2003; Toh et al., 2001; Vink et al., 2001) and environmental factors (e.g. light exposure) (Montaruli et al., 2021; Roenneberg, Wirz-Justice, et al., 2003). Additionally, research has also showed differences in chronotype based on age and sex. For example, children exhibit a morning preference and develop a propensity towards later chronotype during puberty. Around the age of 20, there is a peak "lateness", where the chronotype slowly returns back to morning preference with increasing age (Roenneberg et al., 2004). Sex differences in chronotype emerge at the onset of puberty when increasing levels of sex hormones trigger the development of secondary sex characteristics. It is known that girls tend to go into puberty earlier than boys, and the shift to the puberty-associated late chronotype also happens one year earlier in girls than it does in boys (Hagenauer & Lee, 2012). During puberty, the shift to a later chronotype is more pronounced in boys than in girls (Fischer et al., 2017). This difference remains during the reproductive age: adult men generally tend to have a later chronotype than adult women (Adan & Natale, 2002). The disparity in chronotype between men and women disappears around 40 years of age, coinciding with the perimenopause in women (Fischer et al., 2017; Randler & Engelke, 2019) . A similar pattern is seen in reported sleep duration: after adolescence, women report a longer sleep duration than men (Kocevska et al., 2021), and these sex differences in sleep duration disappear during the perimenopause (Tonetti et al., 2008). The changes in chronotype during puberty and menopause have led researchers to hypothesize that sex hormones could be contributing to shifts in chronotype. This could be explained by the presence of estrogen receptors and androgen receptors in the SCN, which are expressed in a sexspecific way (Kruijver & Swaab, 2002). These also have effects on the circadian system: both androgens and estrogens could affect photic sensitivity within the entrainment pathway in rodents, meaning they can moderate the effect of light- and dark cues on the SCN and thereby modify the circadian system in rodents (Joye & Evans, 2022).
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