Zeitgeber

A zeitgeber (/ˈ(t)saɪtɡeɪbər, ˈzaɪt-/ (T)SYTE-gay-bər, ZYTE-, German: [ˈtsaɪtˌɡeːbɐ]) is any external or environmental cue that entrains or synchronizes an organism's biological rhythms, usually naturally occurring and serving to entrain to the Earth's 24-hour light/dark and 12-month cycles.^[1]^[2]

^ Ehlers, Cindy L.; Frank, E.; Kupfer, D. J. (1988). "Social Zeitgebers and Biological Rhythms". Archives of General Psychiatry. 45 (10): 948–52. doi:10.1001/archpsyc.1988.01800340076012. PMID 3048226.
^ Grandin, Louisa D.; Alloy, Lauren B.; Abramson, Lyn Y. (2006). "The social zeitgeber theory, circadian rhythms, and mood disorders: Review and evaluation". Clinical Psychology Review. 26 (6): 679–694. doi:10.1016/j.cpr.2006.07.001. PMID 16904251.