Chapter 5 138 Florence. In Amsterdam, the medical ethical committee declared that the Medical Research Involving Human Subjects Act (WMO) did not apply to this the data collection of the ENIGI study. All participants received written and oral information, after which they signed informed consent according to the institutions’ guidelines. Participants received GAHT in accordance with Standards of Care of the World Professional Association for Transgender Health (WPATH), edition 7 (World Professional Association for Transgender Health., 2012). The sleep measures (the Insomnia Severity Index (ISI) and Pittsburgh Sleep Quality Index (PSQI)) were collected from July 2017 until October 2021. Prior to the start of analyses, the analysis plan of this study was preregistered at the Open Science Framework (OSF) under DOI 10.17605/OSF.IO/JSCPW. For the purposes of this study we grouped all users of masculinizing GAHT under "trans men" and all users of feminizing GATH under "trans women". We acknowledge and recognize that these particular labels may not represent the personal identities of all participants involved. 2.2. Inclusion criteria and data cleaning Figure 5.1 displays the participant flowchart. Of all ENIGI participants, 467 participants filled out the sleep questionnaires. Participants were excluded if they had undergone gonadal surgery (n=2), had already used GAHT at the baseline measurement (n=5), if their baseline measurement was conducted more than a week after the start of GAHT (n=3), or if they did not start using the prescribed GAHT after the first GAHT appointment (n=5). A small proportion of participating trans women did not use estrogens (n=4) or did not use anti-androgens (n=3), these participants were also excluded from the current analysis. This resulted in a final sample size of 445 participants, of whom 262 trans men and 183 trans women. Due to the ongoing character of the study and the fact that not all centers participated in all measurement time points, the consecutive sample sizes at follow-up measurements differ from the baseline sample size; the respective sample sizes at follow-up are also displayed in the results.
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