Not later, but longer: sleep, chronotype and light exposure in adolescents with remitted depression compared to healthy controls.

The relationship between sleep and adolescent depression is much discussed, but still not fully understood. One important sleep variable is self-selected sleep timing, which is also referred to as chronotype. Chronotype is mostly regulated by the circadian clock that synchronises the internal time of the body with the external light dark cycle. A late chronotype as well as a misalignment between internal time and external time such as social jetlag has been shown to be associated with depressive symptoms in adults. In this study, we investigated whether adolescents with remitted depression differ from healthy controls in terms of chronotype, social jetlag and other sleep-related variables. For this purpose, we assessed chronotype and social jetlag with the Munich ChronoType Questionnaire (MCTQ), subjective sleep quality with the Pittsburgh Sleep Quality Index (PSQI) and used continuous wrist-actimetry over 31 consecutive days to determine objective sleep timing. Given the potentially mediating effect of light on chronotype and depressive symptoms, we measured light exposure with a light sensor on the actimeter. In our sample, adolescents with remitted depression showed similar chronotypes and similar amounts of social jetlag compared to controls. However, patients with remitted depression slept significantly longer on work-free days and reported a worse subjective sleep quality than controls. Additionally, light exposure in remitted patients was significantly higher, but this finding was mediated by living in a rural environment. These findings indicate that chronotype might be modified during remission, which should be further investigated in longitudinal studies.