Zeitgeber

A zeitgeber is any environmental cue that synchronizes an organism's circadian rhythms to the 24-hour cycle. The term, German for "time giver," most strongly refers to the light-dark cycle but can include temperature, feeding times, social cues, and patterns of activity.

In mammals, the central clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus. Light is detected by intrinsically photosensitive retinal ganglion cells containing melanopsin and conveyed to the SCN via the retinohypothalamic tract. The SCN coordinates peripheral clocks and aligns physiology and behavior with the environment. Entrainment keeps the endogenous period near 24 hours, though free-running lengths can vary among individuals and species.

The strength and timing of zeitgebers shape phase shifts. Phase response curves show that light exposure in the early biological night typically advances the clock, while light in the late night or early morning delays it. The range of entrainment depends on zeitgeber intensity and duration as well as the organism's intrinsic period. Other zeitgebers, such as timed meals or social cues, can influence rhythm phase, though their effects are usually weaker than light.

In humans, light is the dominant zeitgeber. Evening light tends to delay the clock, while morning light advances it. Misalignment from jet lag or shift work can disrupt sleep, mood, and performance. Interventions include timed light exposure and darkness, structured schedules, and chronotherapy to manage circadian disorders by manipulating zeitgebers.

Historically, the concept emerged in 20th-century chronobiology through the work of researchers such as Jürgen Aschoff and Colin Pittendrigh, who studied how environmental cues synchronize biological clocks.