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Research into the circadian sleep-wake cycle

Edexcel IAL Psychology · WPS02 Biological Psychology, Learning Theories and Development

The specification asks for research into the circadian sleep-wake cycle (spec 3.1.6). Research can use humans or animals.

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Siffre: isolation from zeitgebers

Michel Siffre spent 63 days about 130 metres underground in Scarasson, a glacial cave in the French Alps, in 1962, with no clock, calendar or daylight. When his surface team told him on 14 September that the planned end date had arrived, he believed it was only 20 August, so he had underestimated the time. His sleep-wake 'day' lengthened from 24 hours to about 24.5 hours. In a later cave stay, about ten years on, his cycle sometimes stretched to about 48 hours (roughly 36 hours awake and 12 asleep).

Conclusion: there is an internal clock that free-runs at slightly over 24 hours; zeitgebers are needed to keep it at exactly 24.

Animal research on the SCN

Ralph et al. (1990): SCN tissue from mutant hamsters with a short (about 20-hour) cycle was transplanted into hamsters whose own SCN had been destroyed. The restored rhythms always had the period of the donor, so the recipients adopted the short cycle. This shows the SCN controls rhythm length.
DeCoursey et al. (2000): 30 wild chipmunks had SCN lesions and were returned to their forest habitat along with 24 surgical controls and 20 intact controls. In the first 80 days after release, significantly more of the lesioned chipmunks were killed by weasels, probably because their disrupted rhythms made them active at the wrong times.

Research on zeitgebers

Campbell and Murphy (1998) shone bright light on the backs of participants' knees at different points in their cycle and measured body temperature and melatonin. The rhythms shifted, and the timing of the light predicted the size and direction of the shift, suggesting light can act as a zeitgeber through the skin. However, Wright and Czeisler (2002) repeated the study while taking extra care that no light reached the eyes and found no effect, so the finding is doubtful.

Evaluation of rhythm research

Isolation studies like Siffre's are case studies, so findings may not generalise; he may also have been affected by artificial light in the cave.
Animal studies allow control (e.g. transplants) that is impossible in humans, but raise ethical issues: DeCoursey's chipmunks suffered harm, and hamsters and chipmunks may differ from humans.
The research is mostly scientific and objective, using measured sleep times.

Worked example

Siffre believed it was 20 August when he emerged on 14 September. How many days had he 'lost'?

Count from 20 August to 14 September.
That is 25 days.

Answer: 25 days.

Key idea: Siffre: free-running cycle slightly over 24 hours without zeitgebers. Ralph: SCN transplant transfers a 20-hour cycle. DeCoursey: SCN loss is dangerous in the wild. Campbell and Murphy: light on skin seemed to shift rhythms, but Wright and Czeisler found no effect.

Check yourself

How many days did Siffre spend in the cave in 1962?
Show answer
63 days. His 1962 stay lasted 63 days.
Siffre emerged on 14 September 1962 but thought it was 20 August. How many days had he underestimated by?
Show answer
25 days. From 20 August to 14 September is 25 days.
What did Ralph et al. (1990) show?
- Light on the knees shifted the sleep cycle
- Transplanting SCN tissue gave SCN-lesioned hamsters the donor's 20-hour rhythm
- Melatonin cures jet lag
- Twins have identical sleep cycles
Show answer
Transplanting SCN tissue gave SCN-lesioned hamsters the donor's 20-hour rhythm. The recipients took on the donor's 20-hour cycle.
Why were the SCN-lesioned chipmunks in DeCoursey et al. (2000) more likely to be killed?
- They could not find food in daylight
- They were kept in a laboratory
- They were awake and active at night, so were more exposed to predators
- Their testosterone levels rose
Show answer
They were awake and active at night, so were more exposed to predators. Disrupted rhythms meant night-time activity.
What did Campbell and Murphy (1998) shine light on?
- The backs of participants' knees
- Participants' closed eyes
- A hamster's SCN
- A cave wall
Show answer
The backs of participants' knees. Light on the skin behind the knees appeared to shift rhythms, though a later replication found no effect.
Which is a weakness of Siffre's cave studies?
- They used animals
- They were laboratory experiments with many participants
- They involved one person, so findings may not generalise
- They had no measure of sleep
Show answer
They involved one person, so findings may not generalise. A single participant limits generalisability.
Evaluate the use of animals in research into the sleep-wake cycle.
Show answer
Animals such as hamsters allow procedures like SCN transplants that would be impossible with humans, giving strong evidence of cause and effect, but the findings may not generalise to humans and the animals can be harmed, which raises ethical issues.. Give a strength and two weaknesses.

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