Chapter 5 152 2019). One small study using sleep EEG measurements in seven trans women found prolonged duration of light sleep after feminizing GAHT use (Künzel et al., 2011). Further studies using sleep EEG measurements could more accurately study changes in sleep duration, and they could also provide novel insights into possible changes in sleep architecture after GAHT. At baseline, participating trans men and women show relatively good selfreported sleep: they report a median sleep duration of 7 hours, a SOL of 20 to 30 minutes, a sleep efficiency above 85% and most participants do not meet the cutoff value for clinically significant symptoms of insomnia. This is remarkable, since transgender people seeking gender-affirming care tend to show high rates of mental health problems (Heylens et al., 2014) and insomnia is an important trans-diagnostic symptom of many psychiatric disorders (Dolsen et al., 2014) . This might also point to a selection bias in our sample. It is possible that the current study has a selection of relatively healthy participants, especially since our sample also shows low rates of psychotropic medication use. Therefore, these baseline results should be interpreted with caution since it is not clear whether the baseline study sample can be generalized to all transgender persons seeking genderaffirming healthcare. Our results do not support the hypothesis that the gender disparity in insomnia and poor sleep as seen in cisgender populations can be explained by sex hormones. However, in this context, it is important to note that there is a difference between endogenous and exogenous hormones. There are two important factors in this context: firstly, whether circulating hormones are within the physiological eugonadal range, and secondly, whether wide fluctuations in hormone levels occur. Most previous studies on sleep and hormones that have been conducted in hypogonadal or perimenopausal persons indicate that strongly fluctuating or clinically low sex hormone levels are associated with insomnia (Ben Dor et al., 2013; Hollander et al., 2001). Reinstatement or suppletion of sex hormones in these groups seems to reduce insomnia symptoms (Shigehara et al., 2018; Silva et al., 2011). This supports the hypothesis that insomnia during hormone changes is mainly caused by hormone levels outside of the physiological healthy range. In our cohort, the protocol of GAHT ensures that
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