General introduction 23 therapy. In men, Leibenluft et al. (1997) found that testosterone suppression resulted in shorter stage N4 sleep and a shorter REM sleep latency compared to testosterone suppression combined with add-back testosterone. However, further experimental studies assessing effects of sex hormones on objective sleep are scarce. 2.3. Sleep in relation to depression Sleep and depression are closely linked to each other: insomnia is a risk factor for depression, and it is also a common symptom during and after depressive episodes (Baglioni et al., 2011), although depressed persons also tend to underestimate their total sleep duration (Difrancesco et al., 2019). Objective sleep measures using sleep EEG also show alterations in depressed persons: SWS is often shorter and more fragmented, and REM sleep is longer and takes place earlier in the night (Steiger & Kimura, 2010). Persons in a depressive episode are more likely to have a later chronotype than persons who are not depressed (Antypa et al., 2016). 3. Exogenous sex hormone use, depression and sleep 3.1. Oral contraceptives and depression The use of OCs could contribute to depressive symptoms, although there might be individual differences in sensitivity for effects of OC (Oinonen & Mazmanian, 2002). Studies found indications of increased risk of depression in younger OC users (De Wit et al., 2020; Skovlund et al., 2016), people who previously had mood-related side effects using OC (Gingnell et al., 2013) and in new users (Johansson et al., 2023). However, there is no conclusive evidence indicating whether depression risk changes after starting OC use, and empirical studies show mixed results. These mixed results could be explained by differences in study samples and setups. One of the likely biases in the study samples in OC studies is a healthy user bias, also sometimes called the survivor bias. The healthy user bias is based on the assumption that side effects of OCs could lead to discontinuation of OC use, resulting in an underestimation of mood effects of OCs in long-time users (Westhoff et al., 2007). Therefore, it is important in
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