Chapter 9 276 use. Furthermore, in Chapter 4 we also assessed differences compared to men and possible underlying associations with cortisol dynamics, which showed a possible effect of sex and OC use on the association between sleep and cortisol dynamics. However, both chapters also have an important limitation in common: these studies examined reported sleep disruptions, but both used one item from a questionnaire that inquired about sleep disruptions using a retrospective multiple-choice question. This limited statistical power and increased risk of retrospection bias, and future studies could improve in this topic by using daily sleep questionnaires or objective sleep measurements. Studies on effects of GAHT on subjective sleep, chronotype, and sleep architecture were described in Chapter 5, 6 and 7. Thus far, knowledge of the effects of GAHT on sleep was scarce, and most studies in this thesis were the first to examine the effects of GAHT on their respective sleep outcomes. Interestingly, in two out of the three chapters, we find changes in sleep which align with sex differences in the cisgender population: in Chapter 6, the changes in the transmasculine GAHT users align with the shorter SWS duration and REM sleep latency in cisgender men compared to cisgender women, although we find no changes in sleep after feminizing GAHT use. In Chapter 7, the earlier chronotype in transfeminine and later chronotype in transmasculine GAHT users align with the later chronotype in cisgender men compared to cisgender women. Although there could be numerous explanations for these findings, they partly support the hypothesis that sex hormones play a role in sex differences in sleep architecture and chronotype. It must be noted, however, that due to the aforementioned intersection of biological and psychosocial changes during GAHT use, as displayed in Figure 9.2, the described effects on sleep (Chapter 5, 6 and 7) could also be due to psychosocial factors (e.g. minority stress, social activities) and physical changes (e.g. hot flashes, night sweats, sleep apnea). There are indications that feminizing GAHT use is associated with hot flashes and night sweats (van Dijk et al., 2019) and masculinizing GAHT use could contribute to a higher risk of sleep apnea (Genzor et al., 2023; Robertson et al., 2019), but these factors were not taken into account in the aforementioned studies. Furthermore, both Chapter 6 and Chapter 7 only examined changes after 3 months of GAHT, leaving unanswered questions on whether these changes remain after longer use of GAHT.
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