Chronotype changes after sex hormone use 197 1. Introduction The human circadian timing system regulates 24-hour rhythms in behaviour and physiological processes. These circadian rhythms are orchestrated by the central brain clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus, which synchronizes a multitude of peripheral clocks throughout the body. The molecular mechanism of the central and peripheral clocks is the transcriptional translational feedback loop of the core clock genes, which has an intrinsic period duration of approximately 24 hours. The molecular clocks regulate the timing of output genes and thus the timing of physiological processes in specific tissues. In the absence of external time cues, the endogenous circadian cycle lasts approximately 24 hours (Roenneberg, Daan, et al., 2003). The duration of the diurnal rhythm of the central clock can, however, differ between individuals due to various factors (e.g. genetics, environmental factors, biological differences), causing some individuals to have circadian rhythms significantly longer or shorter than 24 hours, resulting in a propensity to sleep earlier or later (Roenneberg et al., 2019). This preference in sleep-wake timing based on the intrinsic circadian rhythm is also known as “chronotype” (Adan et al., 2012). People with early chronotypes, ("morning types") rise early, have earlier diurnal peaks in physical and mental performance, and retire early in the evening. Conversely, people with a later chronotype ("evening types") are more inclined to have their time peak activity in the second half of the day and preferably go to bed later (Adan et al., 2012). Previous research has established that people with a late chronotype have a higher risk of sleep problems and poorer mental and physical health (K. S. Jankowski et al., 2019). One potential explanation is that evening-types are more likely to experience sleep loss because they prefer later bed- and wake-up times, but must wake up early to fulfill social obligations. This results in a longer so-called “social jetlag”. Social jetlag is a form of circadian misalignment, caused by a discrepancy between biological sleep timing preferences and social rhythms (K. Jankowski, 2015; Wittmann et al., 2006). Recognizing social jetlag is important since it is associated with an adverse endocrine, behavioral and cardiovascular risk profile due to circadian
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