zeitgeber

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The Word of the Day for September 5 is:

zeitgeber   \TSYTE-gay-ber\   noun
     : an environmental agent or event (as the occurrence of 
light or dark) that provides the stimulus setting or resetting a 
biological clock of an organism

Example sentence:
     Light is known to be a zeitgeber that helps to keep both 
plants and animals on their normal daily and seasonal schedules.

Did you know?
     Zeitgebers are nature's alarm clocks -- both biologically 
and etymologically. The word "zeitgeber" derives from a 
combination of two German terms, "Zeit," which means "time," 
and "Geber," which means "giver," so a "zeitgeber" is literally 
a "time giver." In nature, zeitgebers tend to be cyclic or 
reoccurring patterns that help keep the body's circadian rhythms 
operating in an orderly way. For plants and animals, the daily 
pattern of light and darkness and the warmer and colder 
temperatures between day and night serve as zeitgebers, cues 
that keep organisms functioning on a regular schedule. For 
humans, societally imposed cycles, such as the schedule of the 
work or school day and regular mealtimes, can become zeitgebers 
as well.