Circadian Rhythms

What are biological rhythms?

Distinct patterns of activity in body activity that conform to cyclical time periods. They are influenced by internal body clocks (endogenous pacemakers) as well as external changes to the environment (exogenous zeitgebers).

What are circadian rhythms?

Biological rhythms, subject to a 24-hour cycle, which regulate a number of body processes such as the sleep/wake cycle and changes in core body temperature.

What are all living organisms subject to?

Biological rhythms and these exert an important influence on the way in which body systems behave.

What are all biological rhythms governed by?

Endogenous pacemakers and exogenous zeitgebers. Some of these occur many times during the day (ultradian rhythms) and others take longer than a day to complete (infradian rhythms).

What are two examples of circadian rhythms?

The sleep/wake cycle and core body temperature.

What does the fact that we feel drowsy when it’s nighttime and alert during the day demonstrate?

The effect of daylight - an important exogenous zeitgeber.

What is the sleep/wake cycle governed by?

An internal pacemaker - a biological ‘clock‘ called the suprachiasmatic nucleus - the SCN.

Where is the SCN and what does it do?

It lies just above the optic chiasm which provides information from the eye about light. Light can reset the SCN.

What was Siffre’s cave study?

He was a self-styled caveman who spend several extended periods underground to study the effects of his own biological clock. He was deprived of exposure to natural light and sound, but had access to adequate food and drink.

What were the findings of Siffre’s studies?

• In his first study, he resurfaced in mid September believing it to be mid-August.

• In each case, his ‘free-running‘ biological rhythm settled down to one that was around 25 hours though he did continue to fall asleep and wake up on a regular schedule.

Have other researchers obtained similar results to Siffre?

Other researchers convinced a group of participants to spend four weeks in a WW2 bunker deprived of natural light. All but one participant whose sleep/wake cycle extended to 29 hours displayed a circadian rhythm between 24 and 25 hours.

What do both Siffre’s experience and the bunker study suggest?

That the ‘natural‘ sleep/wake cycle may be slightly longer than 24 hours but that it is entrained by exogenous zeitgebers associated with our 24 hour day such as daylight and meal times.

Despite the findings of those such as Siffre, what should we not overestimate?

The influence of exogenous zeitgebers on our internal biological clock.

What did Folkard et. al study?

A group of 12 people who agreed to live in a dark cave for three weeks, retiring to bed when the clock said 11:45pm and rising when it said 7:45am. Over the course of the study, the researchers gradually sped up the clock (unbeknown to the participants) so an apparent 24-hour day eventually lasted only 22 hours.

What were the findings of Folkard et. al’s study?

It was revealed that only one of the participants was able to comfortably adjust to the new regime.

What does Folkard et. al’s study suggest?

The existence of a strong free-running circadian rhythm that cannot easily be overridden by exogenous zeitgebers.

What are the strengths of research into circadian rhythms>

• Shift work.

• Medical treatment.

How is shift work a strength of research into circadian rhythms?

It provides an understanding of the adverse consequences that occur when they are disrupted (desynchronisation). For example, night workers engaged in shift work experience a period of reduced concentration at around 6 in the morning (a circadian trough) meaning mistakes and accidents ate more likely. Research has also pointed to a relationship between shift work and poor health - shift workers are three times more likely to develop heart disease than those who work more typical work patterns.

What does shift work show?

That it may have real-world economic implications in terms of how best to manage worker productivity.

What is the counterpoint to the shift work?

Studies investigating the effects of shift work tend to use correlational methods. This means it is difficult to establish whether desynchronisation of the sleep/wake cycle is actually a cause of negative effects. There may be other factors. For example, Solomon concluded that high divorce rates in shift workers might be due to the strain of deprived sleep and other influences, such as missing out on important family events.

What does the counterpoint to the shift work suggest about research into circadian rhythms?

This suggests that it may not be biological factors that create the adverse consequences associated with shift work.

How is medical treatment a strength of research into circadian rhythms?

It has been used to improve medical treatments. Circadian rhythms co-ordinate a number of the body's basic processes such as heart rate, digestion and hormone levels. These rise and fall during the course of a day, which has led to the field of chemotherapeutics - how medical treatment can be administered in a way that corresponds to a person's biological rhythms. For example, aspirin as a treatment for heart attacks is most effective if taken last thing at night. Aspirin reduces blood platelet activity, and this can reduce the risk of heart attack. Heart attacks are most likely to occur early in the morning, so the timing of taking aspirin matters.

What does medical treatment show about research into circadian rhythms?

This shows that circadian rhythm research can help increase the effectiveness of drug treatments.

What is the limitation of research into circadian rhythms?

• Individual differences.

How is individual differences a limitation of research into circadian rhythms?

The studies described (Aschoff and Wever, and Siffre) are based on very small samples of participants (just one in the case of Siffre). It seems that sleep/wake cycles may vary widely from person to person. Research by Czeisler et al. found individual differences in sleep/wake cycles varying from 13 to 65 hours. In addition, a study by Duffy et al. (2001) revealed that some people have a natural preference for going to bed early and rising early (known as larks) whereas others prefer the opposite (owls). Even Siffre, in a later 1999 study, observed that his own sleep/wake cycle had slowed down since he was a young man.

What does individual differences suggest about research into circadian rhythms?

This means that it is difficult to use the research data to discuss anything more than averages, which may be meaningless.