Circadian Glossary5 min read
What is a Zeitgeber? The External Clocks That Rule Our Biology
Learn the definition, examples, and biology behind environmental cues that synchronize your master clock.
A Zeitgeber (German for "time giver" or "synchronizer") is any external environmental cue that synchronizes an organism's biological clock to the Earth's 24-hour light/dark cycle.
Without Zeitgebers, the human circadian clock "free-runs" at approximately 24.2 hours, causing sleep times to drift later every day.
Core Examples of Zeitgebers
1. Light (The Master Zeitgeber): Photons hit the retina, sending signals through the retinohypothalamic tract to the suprachiasmatic nucleus (SCN).
2. Temperature: A drop in ambient temperature signals nighttime; a rise signals morning.
3. Food/Feeding Times: Eating resets peripheral clocks in the liver and digestive tract.
4. Exercise/Movement: Muscle activity coordinates peripheral tissue rhythmicity.
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Tip
> To cure jetlag or social shift fatigue, adjust your Zeitgebers simultaneously: get morning light, eat at local times, and exercise during daylight hours.
EditorARC Scientific Team
Date Published2026-06-25
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