Sex hormones, insomnia, and sleep quality: subjective sleep in the first year of hormone use 153 GAHT users are not exposed to hormone levels outside of the healthy physiological range: the endogenous sex hormone levels in hormone users are either supplemented (in the case of trans men, by additional testosterone) or replaced by other exogenous sex hormones (in case of trans women, by replacing testosterone by estrogens). The full serum testosterone and estradiol levels are also reported in the supplementary materials. The fact that our participants do not show clinically significant increases in insomnia after GAHT therefore supports the hypothesis that hormone-related insomnia symptoms are mainly associated with hypogonadal hormone levels. Furthermore, this indicates that reinstatement of hormone levels also prevents insomnia, even when giving back estrogen instead of testosterone as is done in trans women. Altogether, the fact that the participating GAHT users are not exposed to induced hypogonadism could explain why neither groups experience increases in insomnia severity. A second explanation for our findings might also be based on the differences in the dynamics of cisgender persons’ hormonal levels as compared to the hormone dynamics of GAHT users. In cisgender women, levels of estrogen and progesterone fluctuate throughout their menstrual cycle, which has been found to affect subjective sleep quality during certain menstrual phases (Baker & Lee, 2018). In comparison, in trans women the level of estrogen supplementation and androgen suppression is largely constant, although this can still differ based on the administration method of estrogen (e.g. patches or oral administration). This means that trans women likely experience fewer fluctuations in sex hormone levels compared with cisgender women of reproductive age. In cisgender men, testosterone fluctuates daily, whereas in trans men, testosterone levels depend on the administration method of the androgen that is used; the use of daily testosterone gel induces a relatively stable daily testosterone level, but the use of testosterone injections induces 3-week cycle (in the case of testosterone esters) or 12-week cycle (in the case of testosterone undecanoate) of low and high testosterone levels. This means that in testosterone injection users, the level of testosterone fluctuates on a different timescale and in a different manner compared with cisgender men. Based on previous findings that indicate that strong fluctuations in sex hormones are associated with increased insomnia, one could also hypothesize that the amplitude of hormone fluctuations over time irrespective of the type of hormone may be an important determinant of
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