General introduction 21 although important methodological concerns have been raised in these studies (Bhasin & Seidman, 2019). The relationship between endogenous sex hormones and depression is difficult to study, since there seems to be a bidirectional relationship between depression and the HPG axis. In men, some studies find that depression is associated with lower testosterone levels (Hintikka et al., 2009; Westley et al., 2015), although others do not find this effect (de Wit, Giltay, de Boer, Nolen, et al., 2021). This bidirectional association could be due to changes in health and lifestyle during a depressive episode, including weight gain, lack of exercise and poor sleep, all of which could affect testosterone levels (D’Andrea et al., 2020; Su et al., 2021). In women, cross-sectional studies show higher levels of testosterone in those with remitted or current depressive episodes, but testosterone levels were not predictive of future depressive episodes (de Wit, Giltay, de Boer, Bosker, et al., 2021). 2.3. Sleep Sleep is important for general health and well-being, and disturbances of sleep can be caused by or exacerbate mental health problems. There are numerous aspects of sleep that are used to define sleep health, including subjectively reported sleep quality, sleep timing and objectively measured sleep, using polysomnography (PSG), including electroencephalography (EEG). Subjectively reported sleep concerns one’s perception of sleep quality. This can include subjectively reported sleep duration and disruptions, but also perceived trouble falling asleep, staying asleep or not feeling rested in the morning. If these symptoms are frequently present and cause burden on daily life, these could also be classified as insomnia. Sleep can also be measured using objective measures, such as actigraphy or PSG measurements, including EEG. Actigraphy measurements are conducted using a wrist-worn device which estimates sleep length and stages based on patterns of movement. Sleep EEG measurements, which are often considered the golden standard for sleep measurements, are conducted using electrodes that are directly placed on the scalp, recording electrical activity during the night. Sleep timing can be measured by assessing one’s chronotype, which is the natural preference of the body for the timing of wakefulness and sleep, which is assessed by asking about preferred bed-
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