Many patients with depressive disorders experience symptoms in relation to sleep behavior and daily rhythmicity. However, the multifaceted associations between sleep, depression and circadian rhythms are not fully understood. During the past years, the concept of chronotypehas become increasingly popular in research. The Munich Chronotype Questionnaire (MCTQ) derives chronotype from sleep timing on work-free days and therefore represents a biological measure for the circadian clock, whereas the Morningness-Eveningness-Questionnaire(MEQ) assesses chronotype as a subjective preference for different activities at specific times of day. Chronotype changes with age, with adolescents and young adults being especially late types. We conducted a systematic literature research and identified studies that explore the association between chronotype (MEQ, MCTQ) and depressive symptoms or depressive disorders. Most of the studies showed an association between a late chronotype and depressive symptomatology. However, it is still unclear what is cause and effect. We propose a bidirectional relationship: On the one hand, due to reduced social and physical activity, depressed patients get less daylight which causes their chronotype to delay. On the other hand, a discrepancy between internal time (directed by the circadian clock) and external time (such as early school- or works tarting times) can cause problems
Keywords: Chronotype; MCTQ; circadian clock; circadian rhythms; sleep.