Zeitgeber (from German for "time giver") is any external (exogenous) cue that entrains the internal (endogenous) time keeping system of organisms. The strongest Zeitgeber, for both plants and animals, is light. Other, non-photic, zeitgebers include temperature, social interactions, pharmacological manipulation and eating/drinking patterns.
- Aschoff J (1965) The phase-angle difference in circadian periodicity.In "Circadian Clocks" (J. Aschoff, ed.). North Holland Press, Amsterdam, p 262–278.