Biological rhythms: endogenous clocks and zeitgebers
What a biological rhythm is
- A biological rhythm is a change in an organism's activity or physiology that repeats at regular intervals.
- The period is the time taken for one complete cycle.
- Rhythms are classified by their period, and the standard names four:
| Rhythm | Approximate period | Environmental cycle it matches |
|---|---|---|
| Daily (circadian) | 24 hours | Day and night |
| Tidal | 12.4 hours | The rise and fall of the tide |
| Lunar | 29.5 days | The phases of the moon |
| Annual (circannual) | 1 year | The seasons |
- The prefix circa- means "about" — a circadian rhythm has a period of about a day, not exactly 24 hours. That imprecision turns out to be the key evidence for how these rhythms work.
Endogenous and exogenous control
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This distinction is the single most examined idea in this subtopic.
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An exogenous rhythm is driven directly by an external cycle.
- Remove the external cycle and the rhythm stops immediately.
- The organism is simply reacting to conditions as they change.
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An endogenous rhythm is generated by an internal biological clock.
- Remove the external cycle and the rhythm continues, because the clock keeps running.
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The test is always the same, and exam questions are built on it: place the organism in constant conditions — constant light or constant darkness, constant temperature — and see whether the rhythm persists.
- If it stops, the rhythm was exogenous.
- If it continues, the rhythm is endogenous.
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A rhythm running in constant conditions is said to be free-running.
- A free-running rhythm drifts, because the internal clock's period is not exactly 24 hours — it may be 23.6 or 24.8 hours.
- Over successive days the organism's activity starts progressively earlier or later.
- This drift is itself proof the rhythm is internal. An organism reacting to an external cycle could not drift away from it.
Zeitgebers and entrainment
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A clock that drifts would soon be badly out of step with the real world, so it must be reset.
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A zeitgeber (German for "time-giver") is an environmental cue that resets the internal clock.
- The most important zeitgeber is light, specifically dawn and dusk.
- Others include temperature cycles, tidal pressure changes, and food availability.
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Entrainment is the process of the internal clock being reset by a zeitgeber so that it stays synchronised with the environment.
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So a rhythm in nature is the product of both: an endogenous clock generating the rhythm, and an exogenous zeitgeber keeping it accurate.
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Evidence for entrainment: shift an organism's light–dark cycle forward by several hours, and after a few days its rhythm shifts to match. The clock has been reset, not overridden.
How the clock works
- In mammals, the master clock is the suprachiasmatic nucleus (SCN) in the hypothalamus.
- Light detected by the retina signals directly to the SCN, which is how entrainment occurs.
- The SCN controls release of melatonin from the pineal gland. Melatonin is secreted in darkness and suppressed by light, so its concentration is a chemical signal for "night".
- In plants, the clock is a cycle of gene expression within cells, entrained by light detected by phytochrome and cryptochrome pigments.
- In both cases the clock is cell-based and innate — it does not have to be learned.
Why an internal clock is a selective advantage
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The obvious question is why an organism needs a clock at all when it could simply respond to conditions. The answer is anticipation.
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Physiological preparation takes time.
- Enzymes must be synthesised, tissues warmed, hormone levels raised. None of this is instant.
- An organism that begins preparing only when conditions change is already late.
- A clock lets it begin before the change, so it is ready the moment conditions become favourable.
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The cue may be absent or unreliable.
- An animal in a burrow, or a shore organism underwater, cannot see the sky.
- A clock keeps time when the cue is unavailable — cloud, deep water, a long night underground.
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The favourable window may be brief.
- An organism waiting for a signal wastes part of a short window simply detecting it and responding.
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Synchronisation with others.
- A clock shared across a population means individuals become active, or spawn, at the same time, which matters enormously for reproduction.
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Each of these ends in the same currency: more energy gained, less energy wasted, and more successful reproduction than an organism reacting to conditions after they change.
Worked Example
Worked Example
A nocturnal forest invertebrate is kept in a laboratory. Its activity is recorded.
- Days 1–5, normal 12-hour light / 12-hour dark cycle: the animal becomes active within minutes of lights-off each evening.
- Days 6–15, constant darkness: activity continues, but each day it begins about 40 minutes later than the day before.
- Day 16 onward, the light–dark cycle is restored but shifted 6 hours later: within four days, activity again begins at lights-off.
Explain what these results show about the control of this rhythm, and why the response provides a selective advantage.
Answer:
Each phase of the experiment tests something different, so we take them in order.
Days 6–15 show the rhythm is endogenous. In constant darkness there is no external cue to indicate time of day, yet the activity rhythm continued. A rhythm that persists when the environmental cycle is removed cannot be driven by that cycle, so it must be generated by an internal biological clock.
The 40-minute daily drift confirms this and reveals the clock's period. The rhythm is free-running, and drifting 40 minutes later each day means the internal period is about 24 hours 40 minutes — close to a day but not exactly a day. This drift is decisive evidence: an animal merely reacting to an external cue could not drift away from that cue's timing. Only an independent internal oscillator, running slightly slow, produces this pattern.
Day 16 onward shows entrainment. When a light–dark cycle was restored — shifted 6 hours — the animal resynchronised within four days. So light acts as a zeitgeber, resetting the clock rather than driving the rhythm directly. Note that resynchronisation was gradual, not immediate: if the rhythm were exogenous the animal would have matched the new cycle on the very first night. Taking four days shows a clock being corrected in steps.
The full picture. The rhythm is endogenous, generated internally, and entrained by light so it stays accurate. Both components are needed — the clock alone would drift out of step, and light alone could not maintain a rhythm in a dark burrow.
Why this provides a selective advantage.
Being nocturnal is advantageous because it separates the animal in time from visually hunting predators that are active by day, and reduces water loss by restricting activity to the cool, humid hours of darkness.
The clock adds anticipation on top of that:
- The animal can raise its metabolic rate and synthesise digestive enzymes before dusk, so it is ready to forage the moment darkness falls rather than losing the first part of the night preparing.
- It can retreat to shelter before dawn, rather than being caught in the open at first light when predators become active. This matters most: an animal waiting to detect dawn is, by definition, already exposed when it detects it.
- In a burrow or under leaf litter, where light cannot be seen at all, the clock is the only available source of timing information.
More time foraging and less time exposed means more energy gained and a higher chance of surviving to reproduce, so the alleles for an accurate clock increase in frequency.