zeit·geb·er 
(tsītgĕb′ər, zīt-)
(tsītgĕb′ər, zīt-)n.
An external stimulus or cue, such as daylight or a regularly repeated occurrence, that serves to regulate an organism's biological clock.
[German Zeitgeber : Zeit, time; see ZEITGEIST + Geber, giver (from geben, to give, from Middle High German, from Old High German geban; see ghabh- in the Appendix of Indo-European roots).]