English summary 301 faster after 12 months of GAHT and experiencing hot flashes was associated with slightly more insomnia symptoms. In Chapter 6, we examined whether sleep architecture changed after 3 months of GAHT. For this study, we used a specific sleep measurement device to measure sleep duration and depth using single-electrode EEG measurements. Participants of the study wore the sleep measurement device for 7 nights before GAHT, and for 7 nights after 3 months of GAHT, enabling direct comparison of sleep EEG measurements between the time points. Results from this study showed significant changes in sleep patterns in individuals using masculinizing (but not feminizing) hormones. After 3 months of masculinizing GAHT, participants showed slightly shorter slow wave (deep) sleep and slightly longer and earlier REM (dream) sleep. This is in line with sex differences in sleep architecture in cisgender men and women. Lastly, in Chapter 7 we also examined the role of GAHT in participants’ chronotype. A person's chronotype is a person’s natural preference for activity and sleep at earlier or later times, or in other words, whether someone is more of a morning or evening person. There is a difference in cisgender men and women, with men showing a later chronotype than women, which includes preferring a later bedtime. In our study, results show that participants who used masculinizing hormones went to bed about 24 minutes later, and participants who used feminizing hormones went to bed about 21 minutes earlier after 3 months of GAHT. These results align with sex differences seen in cisgender populations, indicating that GAHT might shift chronotype towards users’ gender identity. What are the effects of sex hormones on the association between depression and sleep? In Chapter 8, we conducted a systematic review to examine whether sex hormones could affect the association between sleep and depression. Our findings show that studies on sex hormones and depression were scarce, and study setups showed large differences. Overall, studies in healthy populations and studies with depressed participants showed associations between depressive symptoms and sleep. However, in studies with participants with premenstrual dysphoric disorder or studies on participants
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