English summary 297 English Summary In this thesis, I have examined the effects of sex hormones on depression and sleep. I examined whether the use of sex hormones, specifically oral contraceptives (OCs) and gender-affirming hormone therapy (GAHT), affected depressive symptoms, sleep quality, sleep architecture and chronotype. The main research questions in this thesis were: 1. Are OC use and GAHT use associated with changes in depression? 2. Are OC use and GAHT use associated with changes in sleep? 3. What are the effects of sex hormones on the association between depression and sleep? Background Depression is twice as common in women compared to men, and the risk of insomnia is 1.5 times higher in women than in men during the reproductive age. It has been suggested that sex hormones may partly explain this difference in prevalence. Changes in these hormones, especially in estrogen and progesterone, may possibly contribute to more depressive symptoms and more insomnia. Alternatively, it is possible that testosterone protects against depression and insomnia. The prevalence of depressive symptoms and insomnia increase during life phases when sex hormones fluctuate, such as during puberty, pregnancy and around the menopause, the prevalence of depression and insomnia increase. There is a paradoxical sex difference in objectively measured sleep: although women show a higher prevalence of self-reported insomnia, they also show better sleep architecture than men. Studies using sleep electroencephalography (EEG) show that women have longer sleep, they have more slow wave (deep) sleep and they wake up at night less than men. However, it is difficult to examine the causal role of sex hormones in the sex differences in depression and sleep. One way to examine the effects of sex hormones on depression and sleep is to investigate situations where individuals start taking exogenous hormonal medications. Therefore, this study aimed to examine the effects of oral contraceptives (OCs) and the effects of gender-affirming hormone therapy (GAHT). Research on these hormones can serve a dual purpose: firstly, these
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