Chapter 6 182 One explanation for the sex discrepancy in our results, showing that transmasculine participants show altered sleep but transfeminine participants do not, could also be found in possible sex differences in sex hormone sensitivity in the brain. The organizational-activational model of sexual differentiation (as posed by Phoenix et al. (1959)) states that sex hormones can permanently affect neural development (so-called organizational effect), and they can also have a direct effect on neural activity (so-called activational effect). Studies in rodents show that this model might also apply for sex hormone effects on sleep: manipulation of sex hormone exposure during development in rats also alters the sensitivity to sex hormone effects on sleep. Male rats that underwent neonatal castration, resulting in lack of exposure to androgens, were found to show larger sleep changes after treatment with estradiol and progesterone than male rats that underwent endogenous puberty and were therefore exposed to androgens (Branchey et al., 1973). Vice versa, female rats that were treated with masculinizing postnatal testosterone treatment showed no sleep changes after sex hormone manipulations, whereas the untreated females which were exposed to endogenous puberty and female sex hormone exposure showed significant sleep changes after sex hormone changes (Cusmano et al., 2014). These findings indicate that exposure to female sex hormones or lack of exposure to male sex hormones could possibly alter the brain’s sensitivity to sex hormone changes. Interestingly, the demographic characteristics show that sleep quality improved and insomnia symptoms reduced after 3 months of GAHT in the TM group, although the found effect sizes are small to medium (ISI: 0.37 and PSQI: 0.39). This is in line with previous work from our group in a different cohort, where we find that insomnia and sleep quality both improve after 3 months of GAHT use, but show no lasting improvements after 12 months of GAHT use (Morssinkhof et al., 2023). Within the current study, we seem to reproduce the existing paradox in the general population, where cisgender men show poorer sleep quality when measured using objective measures, but they report better sleep in subjective measures. Although there are no definitive explanations for this paradox, it is sometimes assumed that subjective and objective sleep are affected by different factors. For example, perceived sleep quality and insomnia symptoms could be more affected by anxiety and depression, which are more prevalent in women (Kessler, 2003),
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