Chapter 7 208 without PCOS (Karasu et al., 2021). Estrogen had the opposite effect: previous research found that estrogen has a phase-advancing effect in rodents, meaning chronotype shifts to an earlier preference (Albers et al., 1981; Leibenluft, 1993). Thus, previous research indicated that the effects of estrogen and testosterone on chronotype might be opposing, which is in line with our current findings. Based on our findings it is not possible to separately assess the role of testosterone and estrogen in the reported chronotype changes. All TF participants used both estrogens and antiandrogens, and therefore the resulting earlier chronotype could both be caused by the increase in estrogen signaling, the decrease in testosterone signaling, or by an interaction of both factors. A similar limitation is found in the TM participants: although they all start using testosterone and the serum testosterone levels strongly increase, many also report cessation of their menstrual cycle, which most likely means that the endogenous estradiol fluctuations have also stopped. Furthermore, some of the TM participants used cycle regulation, which also affects gonadal hormone levels. Therefore, although our results do show that exogenous use of gender-affirming hormones changes chronotype in line with cisgender sex differences, it is not possible to assess which hormonal mechanisms are underlying these shifts in chronotype. Concerning sleep duration, no changes were found in the TF participants after 3 months of GAHT. This is in contrast with the findings of Liu et al., (2003), who found that men show a sleep duration reduction of approximately 1 hour after administration of a high dosage of testosterone, and Sakaguchi et al., (2006) who discovered that short sleepers had higher testosterone levels. Furthermore, we did not find any change in sleep duration in the TF participants after 3 months of hormone therapy with estrogens and antiandrogens. These findings may indicate that hormone therapy with either testosterone or estrogen and antiandrogens might not directly affect reported sleep duration. The strengths of this study lie mainly in its unique study population and its prospective study setup. Firstly, the study of transgender hormone users enables us to study the effects of exogenous sex hormones, which are administered in such a dosage that the sex hormone levels in our participants transition from the levels found in cisgender women towards the levels found in cisgender men, or vice versa, as shown in Table 1. This is
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