(2026 Exams) Studies on Biological Rhythms (A2)

**Menaker et al (1978)** investigated the importance of the SCN in regulating circadian rhythms.

**Siffre (1975)** investigated the influence of light as an exogenous zeitgeber on circadian rhythms.

**Czeisler et al (1982)** wanted to find out whether rotating shift patterns based on circadian principles could improve sleep patterns.

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Menaker et al (1978) - Structure in Hamsters

Menaker et al (1978) investigated the importance of the SCN in regulating circadian rhythms.

Menaker et al (1978) bred a strain of hamsters so that they had abnormal circadian rhythms of 20 hours rather than 24.
SCN neurons from these abnormal hamsters were then transplanted into the brains of normal hamsters.

These normal hamsters then displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the hamster.
Further confirmation of the importance of the SCN came in the reverse experiment, planting SCN neurons from normal hamsters into the brains of abnormal hamsters. Rather than maintaining their abnormal circadian rhythm, the recipient hamsters changed to a circadian pattern of 24 hours.
This shows the importance of the SCN in regulating circadian rhythms.

Siffre (1975) investigated the influence of light as an exogenous zeitgeber on circadian rhythms.

A case study.
Michel Siffre lived underground to study his own circadian rhythms.
While living underground he had no external cues to guide his biological rhythms – no daylight, no clocks or radio.
He simply woke, ate, and slept when he felt it was appropriate to do so. The only thing influencing his behavior was his internal body clock.

After his first underground stay of 61 days in the Southern Alps in 1962, he resurfaced on the 17th September believing it to be August 20th, showing his circadian rhythm to be around 26 hours.
On the second occasion, he spent 6 months in cave in Texas. His natural circadian rhythm settled down to just over 24 hours but with some dramatic variations.

On his final underground stay in 1999, he was interested in the effects of ageing on circadian rhythms. He found that his body clock ticked more slowly compared to when he was a young man (he was 60 at this point), sometimes stretching his circadian rhythms to 48 hours.

This shows that whilst the natural circadian rhythm of human beings is around 24 hours, light is important to reset the natural rhythm.

Strengths
- Controlled environment – could be replicated.
Limitations
- Ecological validity – most of us do not live underground.
- Cannot generalise from a case study.

Czeisler et al (1982) wanted to find out whether rotating shift patterns based on circadian principles could improve sleep patterns.

A field experiment with 85 male workers from a Utah chemical plant on a rotating shift pattern and a control group of 68 male workers on non-rotating day shifts and swing shifts. They were matched for age and comparable jobs.
The rotating shift pattern was three 8-hour shifts which were 12am-8am, 4pm-12am and 8am-4pm.
33 workers were on the normal cycle of shift change every 7 days, 52 workers changed shifts every 21 days. All the workers completed self-reports on sleep, health and shift preferences. Productivity was analysed after 9 months.

Results
- Workers on rotating shifts did report more sleep problems. However, the delayed shift change reduced these issues and respondents did prefer it. Productivity increased and staff turnover decreased.
Conclusion
- Shift patterns that rotate by phase delay with an extended interval between each rotation seem to suit the human circadian rhythm better and reduce sleep issues. However, a 21-day cycle does have implications for social and family lifestyles.