CLOCK and major depressive disorder: Differences in chronotype between females and males as well as among different age groups may also have non-hormonal causes, e. g., differences in family life (e. g., typically-‘female’ responsibilities such as household chores and childcare), work regimes (e. g., shift work) as well as somatic and mental disorders (e. g., depression [14]), that may feed back onto the circadian clock via modification of internal bodily processes and/or light-dark exposure (‘zeitnehmer loop’ [42]).