From the German for “time giver,” it refers to any external cue that can entrain (or reset) the time-keeping system of organisms. In humans, this circadian system
, or biological clock
, is located in the suprachiasmatic nucleus within the hypothalamus
of the brain. These cues follow a periodic pattern, and one of the strongest cues is the pattern of light and darkness.
Light is the principal zeitgeber that controls the biological clock.