Circadian rhythms

Circadian Rhythms (AO1)

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What is a circadian rhythm?

A type of biological rhythm that completes one full cycle every 24 hours, for example the sleep-wake cycle

What two factors affect circadian rhythms?

Endogenous pacemakers and exogenous zeitgebers

What are exogenous zeitgebers?

External changes in the environment which affect or entrain our biological rhythms

What are endogenous pacemakers?

Internal body clocks that regulate many of our biological rhythms, such as the influence of the SCN on the sleep-wake cycle

Suprachiasmatic Nucleus — SCN (AO1)

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What is the suprachiasmatic nucleus (SCN) and where is it found?

The master circadian pacemaker found in the hypothalamus, which synchronises body clocks found in all cells of the body

Why does the SCN need to be constantly reset?

So that our bodies remain in synchrony with the outside world

What is entrainment and what causes it?

The process by which light (an exogenous zeitgeber) sets the body clock to the correct time, keeping our circadian rhythms aligned with the outside world

The Sleep-Wake Cycle (AO1)

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What controls the sleep-wake cycle?

The SCN, which determines when we need to sleep and when we need to wake up

When does our strongest drive to sleep occur?

Between 2-4am and 1-3pm

How does light information reach the SCN?

Information about light levels is sent from the eyes via the optic chiasm to the SCN

What happens when the SCN detects darkness?

It sends a message to the pineal gland to produce melatonin, which causes brain activity to slow and induces sleep

What happens when the SCN detects daylight?

It causes the pineal gland to stop producing melatonin, brain activity increases and wakefulness is prompted

What happens to the SCN each morning?

It is reset or entrained

What happens to the sleep-wake cycle in the absence of external cues such as light?

The SCN is described as free-running and will maintain a cycle of approximately 24-25 hours

What can trigger desynchronisation of the sleep-wake cycle?

Changes in light exposure

Research — Siffre (1962) (AO1/AO3)

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What was the procedure of Siffre's (1962) study?

Siffre descended into a cave on 16th July 1962, completely devoid of natural light, and finished his experiment on 14th September, believing it to be 20th August

What did Siffre's (1962) study find about the sleep-wake cycle?

His sleep-wake cycle did not conform to a 24-hour period but was around 24 hours and 30 minutes (24.9 hours), and there was a disconnection between psychological time and the clock

What was the conclusion of Siffre's (1962) study?

It is evidence of a free-running circadian rhythm of just over 24 hours, one that is not affected by exogenous zeitgebers, demonstrating an internal clock independent of the natural day-night cycle

Evaluation (AO3)

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Supporting Evidence — Aschoff and Wever (1976) (AO3)

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What did Aschoff and Wever (1976) find in their study?

Participants who spent 4 weeks in a WWII bunker deprived of natural light mostly displayed a circadian rhythm between 24 and 25 hours, supporting Siffre's findings

What do both Siffre and Aschoff and Wever's studies conclude?

That the natural sleep-wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers such as daylight and mealtimes associated with our 24-hour day

Contradictory Findings — Folkard et al. (1985) (AO3)

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What should we be cautious of when considering the influence of exogenous zeitgebers?

We should not overestimate the influence of exogenous zeitgebers such as light on our internal biological clock

What was the procedure of Folkard et al.'s (1985) study?

12 participants lived in a dark cave for 3 weeks, going to bed when the clock said 11.45pm and waking at 7.45am, but unbeknown to participants, researchers gradually sped up the clock so an apparent 24-hour day eventually lasted only 22 hours

What did Folkard et al. (1985) find?

Only 1 participant could comfortably adjust to the new regime, suggesting the existence of a strong free-running circadian rhythm controlled by the SCN that cannot easily be overridden by exogenous zeitgebers

Confounding Effect of Artificial Light (AO3)

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How can artificial lighting affect circadian rhythms?

It can create shifts in circadian rhythms by up to 6 hours

What was the problem with Siffre's use of artificial light in his study?

Siffre's research was conducted when researchers believed artificial light had no effect on biological rhythms, but over 2 months Siffre could have entrained his own circadian rhythm by using the light to signal sleeping and waking times

What does the confounding effect of artificial light suggest about Siffre's conclusions?

That the conclusions made about his free-running circadian rhythm may not be entirely accurate

Practical Application (AO3)

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What practical application does research into circadian rhythms have for shift workers?

Night workers experience a circadian trough around 6am where concentration is reduced, meaning mistakes and accidents are more likely (Boivin et al., 1996)

What health consequences are associated with shift work?

Shift workers are three times more likely to develop heart disease than people who work more typical patterns

What real-world implications does research into the sleep-wake cycle have?

It may have economic implications in terms of how best to manage worker productivity by understanding the adverse consequences of circadian disruption