Sex hormones, insomnia, and sleep quality: subjective sleep in the first year of hormone use 137 report that they experience poor sleep (Auer et al., 2017). However, up till today the effect of GAHT on insomnia and sleep quality has not yet been studied. Therefore, this study will prospectively assess self-reported insomnia symptoms and sleep quality during GAHT use in transgender persons. This study firstly aims to study the trajectory of insomnia symptoms and sleep quality during the first 12 months of GAHT. Secondly, it aims to assess changes in sleep onset latency, total sleep duration, and sleep efficiency after 12 months of GAHT. Thirdly, as hot flashes are a common side effect of estrogens and anti-androgens (van Dijk et al., 2019), we explore whether hot flashes play a moderating role in the relation between GAHT and insomnia. Based on differences in insomnia and sleep quality in the cisgender population, we hypothesize that after 12 months of GAHT, trans men will report fewer insomnia symptoms and better sleep quality, and trans women will report more insomnia symptoms and worse sleep quality. We also hypothesize that participants who experience hot flashes will report more insomnia symptoms. 2. Methods 2.1. Study participants Transgender persons who participated in the endocrinological part of the European Network for the Investigation of Gender Incongruence (ENIGI) study were included. ENIGI is an ongoing prospective cohort study that aims to investigate the clinical effects and side effects of GAHT in Oslo (Oslo University Hospital), Ghent (University Hospital Ghent), Tel Aviv (Souraski Hospital), Florence (University Hospital Florence), and Amsterdam (Amsterdam University Medical Center)(Dekker et al., 2016). For the current analysis, data was collected from the centers in Ghent, Tel Aviv, Florence, and Amsterdam. Participants were included at the start of GAHT if they were aged 17 or older (Ghent) or aged 18 years or older (Tel Aviv, Florence, and Amsterdam), native in their national language (Flemish, Hebrew, Italian, or Dutch) and had a confirmed diagnosis of gender dysphoria according to the DSM-4 or DSM-5, depending on when they were diagnosed. Ethics approval was granted by the local medical ethical committees in Ghent, Tel Aviv, and
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