Chapter 6 168 (Söderpalm et al., 2004) but also reduced slow wave sleep duration in males (Friess et al., 1997). Administration of a progesterone antagonist in healthy males resulted in increased sleep onset latency and more time spent awake during the night (Wiedemann et al., 1998). One of the limitations in the field of sex hormone use is that most studies that examine have mainly focused on effects of short-term hormone administration, ranging from days to weeks of hormone use, and hormone use within the same sex (e.g. use of estrogens and progestins in females, or testosterone in males). Furthermore, it is difficult to examine the causal role of sex hormone use in groups with sex hormone conditions, including peri- and postmenopausal females, hypogonadal males and females with polycystic ovary syndrome. These conditions are already known to affect sleep, and therapy in the form of sex hormone use could restore healthy hormone levels and therefore improve sleep (Shaver & Woods, 2015; Shigehara et al., 2018; Teo et al., 2022). This makes it difficult to disentangle whether the observed sex differences in sleep architecture in the population are driven by sex hormones, or by other biological sex-specific factors like sex chromosomes or developmental sex differences. One way of gaining insight into the effects of sex hormones on human sleep would be to study the effects of male sex hormones (e.g. testosterone) in females and the effects of female sex hormones (e.g. estrogen) in males, but this is difficult to do in an experimental setting. Transgender users of gender-affirming hormone therapy (GAHT) use sex hormones to masculinize or feminize their bodies, to better align their physical state with their gender identity (Cocchetti et al., 2022). The study of changes in sleep architecture after GAHT use can offer insight into the direct effect of sex hormone use on sleep architecture. The use of GAHT in transfeminine persons assigned male at birth most often consists of antiandrogens and estrogens which act to feminize the body, and the use of GAHT in transmasculine persons assigned female at birth consists of testosterone use which acts to masculinize the body. Since GAHT use commonly results in sex steroid levels comparable to the opposite sex, this enables us to specifically examine the effects of healthy physiological levels of sex hormones on sleep.
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