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title: Circadian Rhythm
modified: 2026-08-04
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= 生理時鐘 = 晝夜節律

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> Circadian rhythm is the conductor of your body’s orchestra/symphony.

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# What is Circadian Rhythm?

The circadian rhythm is an internal biological clock that governs/modulates the sleep-wake cycle. It operates on a roughly 24-hour cycle, _responding primarily to light and darkness in the environment_.

## Key Features

* Generated from within, but synchronized (entrained) by external cues [^1], especially light.
* Found in almost all living organisms, from plants to animals to humans.
* Can be shifted by travel ([jet lag](jet-lag.md)), shift work, or changes in [routine](be-a-habit-and-routine-machine.md), but prefers regularity.
* **Not exactly 24 hours:** The intrinsic period of the human clock averages about **24.2 hours** — slightly longer than a day. Without external cues to reset it (a state called _free-running_), the clock drifts later each day. This is precisely why daily time cues matter so much.
* Loves **consistency**. It can adapt to almost any schedule — even a “night owl” one — provided the schedule is _regular_.

## Examples

* **[Morning sunlight](sunlight-viewing.md)** triggers a rise in [cortisol](cortisol.md), helping you feel alert and awake.
* **Body temperature** drops to its lowest point ([temperature minimum](temperature-minimum.md)) a few hours before waking, then rises through the day.
* **Melatonin** (the [sleep](sleep.md) hormone) increases in the evening as it gets dark, signaling the body to prepare for sleep.
* **[Adenosine](adenosine.md)** builds up during wakefulness, creating sleep pressure, and is cleared during sleep.

[^1]: _[zeitgeber](https://www.google.com/search?q=zeitgeber)_ (German for “time-giver”)