word of the day

zeitgeber \TSAHYT-gey-ber\  , noun:
An environmental cue, as the length of daylight, that helps to regulate the cycles of an organism's biological clock.

The light–dark transition Zeitgeber is widely used by plants to set internal clocks not just for leaf movement but for many other activities as well.
-- John King, Reaching for the Sun

The most prominent zeitgeber in humans is the light/dark cycle.
-- Harold R. Smith, Cynthia Comella, Birgit Högl, Sleep Medicine

Zeitgeber comes directly from the German word which literally means "time-giver." It entered into English in the 1970s.