zeitgiver

Zeitgeber is a term used in chronobiology to describe an external cue that synchronizes an organism’s internal circadian clock to the 24-hour day. The word is German for “time giver.” The standard spelling in most scientific literature is zeitgeber; zeitgiver is a less common variant that can appear as a misspelling.

Primary zeitgebers include the light-dark cycle, which is the dominant cue for most organisms. Light information received by the retina is transmitted to the suprachiasmatic nucleus (SCN) in the brain, where it can reset the timing of the internal clock. Other environmental cues can also serve as zeitgebers, such as regular meal times, temperature fluctuations, social interactions, and activity schedules. These non-photic cues can influence clock phase, especially when light input is weak or inconsistent.

The function of zeitgebers is to entrain the internal clock to the external environment, promoting alignment of physiological processes—sleep-wake cycles, hormone secretion, metabolism, and body temperature—with the day-night cycle. When zeitgebers are absent or irregular, rhythms may free-run with a period slightly longer or shorter than 24 hours.

Applications and implications include understanding jet lag, shift work, and the effects of screen exposure on sleep. In research, zeitgebers are manipulated to study entrainment, circadian timing, and the adaptability of organisms to environmental change. See also circadian rhythm, entrainment, and phase shifting.