Chronotype changes after sex hormone use 199 Thus far, human research has mainly focused on the association between sex hormones and insomnia (Morssinkhof et al., 2023), while studies on sex hormones and chronotype are scarce. Studies that have examined the association between sex hormones and chronotype show indications that higher testosterone is associated with later chronotypes. Randler et al. (2012) and Jankowski et al. (2019) both found associations between testosterone levels and later chronotypes in males. However, Yuan et al. (2023) found that only in women, and not in men, free testosterone was associated with a later chronotype. These studies all assessed crosssectional associations, and it is therefore still unknown whether the sex hormones could also have a causal effect on the sex differences in chronotype in humans. Transgender individuals who use gender-affirming hormone therapy (GAHT) are a unique group in whom we could prospectively study effects of sex hormone use on chronotype. Transmasculine (TM) persons, who were assigned female at birth and desire masculinization, can use testosterone, whereas transfeminine (TF) persons, who were assigned male at birth and desire feminization, can use estrogens and anti-androgens. GAHT has as an effect on multiple homeostatic bodily processes and causes significant changes in physical appearances towards the phenotype of the other sex (Cocchetti et al. 2022), but their effects on chronotype are not yet known. This study aims to examine the effect of sex hormones on chronotype, specifically the midpoint of sleep and sleep duration, in transgender persons after 3 months of GAHT use. We hypothesize that TM persons, using testosterone, show a change in chronotype from early to later-type and a shorter sleep duration after 3 months of GAHT, whereas TF persons, using estrogen and anti-androgens, show a chronotype moving from a later to earlier chronotype and a longer sleep duration after 3 months of GAHT. 2. Methods 2.1. Study population Participants for this study were recruited from the Relationship between Emotions and Sleep in Transgender persons: Endocrinology and Depression (RESTED) study, a prospective cohort study investigating the effects of
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