The circadian sleep-wake cycle: pacemakers and zeitgebers

Internal pacemakers (endogenous)

• The suprachiasmatic nucleus (SCN) is a small cluster of nerve cells in the hypothalamus, just above the optic chiasm. It is the main body clock in mammals.
• The SCN receives information about light from the eyes and passes it to the pineal gland.
• In darkness the pineal gland produces more melatonin, a hormone that makes us sleepy; light suppresses melatonin.

Without any external cues the cycle 'free-runs' at slightly more than 24 hours.

External zeitgebers (exogenous)

Zeitgebers ('time givers') are external cues that reset the body clock each day (entrainment).

• Light is the main zeitgeber. It resets the SCN through the eyes; Campbell and Murphy (1998) suggested light on the skin may also have an effect.
• Social cues such as mealtimes and bedtimes. Babies start with random sleep patterns and become entrained over the first months.

The pacemaker and zeitgebers normally work together.

Variations and disruption

When zeitgebers suddenly change, the internal clock takes time to catch up:

• Jet lag: flying across time zones puts the body clock out of step with local light and social cues.
• Shift work: working at night means sleeping when light and social cues signal day.

These disruptions can cause tiredness, poor concentration and mood changes, showing the clock cannot reset instantly.

Evaluation

• Strength: animal research (Ralph et al., 1990) shows the SCN directly controls rhythm length.
• Strength: practical applications, such as light exposure and melatonin to reduce jet lag.
• Weakness: much evidence comes from animals, so generalisation to humans is limited.
• Weakness: isolation studies, such as Siffre's, use very few participants.
• Weakness: it is hard to study one zeitgeber alone, because they interact.

Worked example

Explain why a person feels sleepy in the evening.

1. As light fades, the eyes send less light information to the SCN.
2. The SCN signals the pineal gland, which increases melatonin production.
3. Melatonin makes the person sleepy.

Answer: Darkness leads the SCN and pineal gland to release melatonin, causing sleepiness.

Check yourself

1. Where is the suprachiasmatic nucleus?
   • In the spinal cord
   • In the hypothalamus
   • In the occipital lobe
   • In the amygdala
   Show answer

   In the hypothalamus. The SCN is in the hypothalamus above the optic chiasm.

2. Which hormone does the pineal gland release in darkness?
   • Testosterone
   • Cortisol
   • Adrenaline
   • Melatonin
   Show answer

   Melatonin. Melatonin rises in darkness and promotes sleep.

3. Which is an exogenous zeitgeber?
   • The SCN
   • Daylight
   • The pineal gland
   • Melatonin
   Show answer

   Daylight. Zeitgebers are external cues; light is the main one.

4. Resetting the body clock by external cues is called
   • extinction
   • free association
   • entrainment
   • reciprocal inhibition
   Show answer

   entrainment. Entrainment means the rhythm is set by zeitgebers.

5. Jet lag happens because
   • the body clock is out of step with new local zeitgebers
   • melatonin stops being produced
   • the SCN is damaged by flying
   • light is no longer a zeitgeber
   Show answer

   the body clock is out of step with new local zeitgebers. The internal clock takes days to re-entrain.

6. A free-running rhythm lasts 24.5 hours. How many hours later would a person wake after 6 days without cues, compared with a 24-hour schedule?
   Show answer

   3 hours. 6 x 0.5 = 3 hours later.

7. Explain how internal pacemakers and external zeitgebers work together to control the sleep-wake cycle.
   Show answer

   The SCN acts as the internal body clock and controls melatonin release from the pineal gland, while light and social cues act as zeitgebers that reset the SCN each day so the cycle stays at 24 hours.. Internal clock plus external resetting.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification, Issue 5 (February 2021)
• Pearson IAL Psychology component guide: WPS02 (Issue 2, November 2024)
• SimpleStudy: Endogenous pacemakers and exogenous zeitgebers

Last updated 8 November 2026. Spotted a mistake? Tell us.