In chronobiology, entrainment of a circadian system is the alignment of its own period and phase to the period and phase of an external rhythm. A common example is the entrainment of endogenouscircadian rhythms (which in mammals are generated by the suprachiasmatic nuclei of the hypothalamus) to the daily light-dark cycle. Of the several possible cues, called zeitgeber (German for time-giver, synchronizer), which can contribute to entrainment, bright light is by far the most effective.
References
Pittendrigh CS (1981) Circadian systems: Entrainment. In Handbook Behavioral Neurobiology, Vol. 4. Biological Rhythms, J. Aschoff, ed. pp. 239-68, University of California Press, New-York.
