zeitgeber

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 recurring 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.