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Laboratory studies have shown that meaningful changes in light exposure lead to phase shifts in markers of the circadian clock. In natural settings, light is a complex signal varying with external conditions and individual behaviors; nonetheless, phase of entrainment is assumed to be fairly stable. Here, we investigated the influence of season and weekly schedule (as indicators of variation in light landscapes) on phase of entrainment. Using a within\u2010subjects design (N\u00a0=\u00a033), we assessed dim\u2010light melatonin onset (DLMO) as a circadian phase marker in humans, on workdays and work\u2010free days, in summer (under daylight saving time) and in winter, while also estimating sleep times from actimetry. Our mixed\u2010model regressions show that both season and weekly structure are linked with changes in phase of entrainment and sleep. In summer, both DLMO and sleep times were about 1\u00a0hour earlier compared to winter, and sleep duration was shorter. On work\u2010free days, DLMO and sleep times were later, and their phase relationship differed more relative to workdays. All these effects were stronger in later chronotypes (those who habitually sleep late). Our results confirm that phase of entrainment is earlier when stronger zeitgebers are present (summer) and show that it relates to midday or midnight rather than sunrise or sunset. Additionally, they suggest that late chronotypes are capable of rapid phase shifts each week as they move between workdays and work\u2010free days, stimulating interesting questions about the stability of circadian phase under natural conditions.<\/jats:p>","DOI":"10.1111\/jpi.12723","type":"journal-article","created":{"date-parts":[[2021,2,20]],"date-time":"2021-02-20T04:02:24Z","timestamp":1613793744000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":75,"title":["Weekly, seasonal, and chronotype\u2010dependent variation of dim\u2010light melatonin onset"],"prefix":"10.1111","volume":"70","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-5348-9212","authenticated-orcid":false,"given":"Giulia","family":"Zerbini","sequence":"first","affiliation":[{"name":"Institute of Medical Psychology Faculty of Medicine LMU Munich Munich Germany"},{"name":"Groningen Institute for Evolutionary Life Sciences (GELIFES) University of Groningen Groningen The Netherlands"},{"name":"Department of Medical Psychology and Sociology Faculty of Medicine University of Augsburg Augsburg Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0717-9432","authenticated-orcid":false,"given":"Eva C.","family":"Winnebeck","sequence":"additional","affiliation":[{"name":"Institute of Medical Psychology Faculty of Medicine LMU Munich Munich Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Martha","family":"Merrow","sequence":"additional","affiliation":[{"name":"Institute of Medical Psychology Faculty of Medicine LMU Munich Munich Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2021,3,11]]},"reference":[{"key":"e_1_2_10_2_1","volume-title":"Circadian Clocks: Proceedings","author":"Aschoff 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