CIRCADIAN RHYTHMS

→ Biological rhythms are cyclical cycles in patterns of body activity

→ Biological rhythms are governed by:

Endogenous Pacemakers → internal body clocks

Exogenous Zeitgebers → external cues in the environment

Circadian rhythms last for around 24 hours

Examples of Circadian rhythms:

• The sleep-wake cycle

• The core body temperature cycle

~The Sleep-Wake Cycle~

→ Light is an important exogenous zeitgeber in the sleep-wake cycle

→ We feel alert when it is light, and tired when it is dark

→ Without the influence of light, endogenous pacemakers dictate sleep-wake cycle (this is known as ‘free-running’)

→ Siffre studied the sleep-wake cycle by spending several extended periods of time in caves underground

→ During these periods of time, he had no exogenous zeitgebers to guide his sleep-wake cycle (no natural light, no sounds, no clocks etc)

→ Instead his sleep-wake cycle was dictated by his endogenous pacemakers (he was therefore ‘free-running’)

→ In each case, Siffre found that his ‘free-running’ sleep-wake cycle may be slightly longer than 24 hours

~The Core Body Temperature Cycle~

→ Core body temperature varies by about 2 degrees celsius during a 24-hour period

→ It is at its lowest at around 4am (36degrees)

→ It is at its highest at around 6pm (38degrees)

→ When the core temperature begins to drop after 6pm, this promotes a feeling of tiredness

→ When the core temperature begins to rise at 4am, this promises a feeling of alertness

→ There is aso a small drop in body temperature between 2pm and 4pm, explaining why many people feel tired in the afternoon

→ Gupta (1991) found that participants performed better on IQ tests when they were assessed at 7pm as opposed to 2pm and 9am

→ Core body is at its highest during the early evening, suggesting that the warmer we are, the better our cognitive performance

EVALUATION

Research Support:

→ SHIFT WORK

• One strength of research into circadian rhythms is that 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 around 6 in the morning (a circadian trough) meaning mistakes and accidents are more likely (Boivin et al 1996)

• Research has also suggested a relationship between shift work and poor health: shift workers are X3 more likely to develop heart disease than people who work normal day patterns.

• This shows that research into circadian rhythms may have real-world economic implications in terms of how best to manage worker productivity.

• However, 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 these observed negative effects.

• For example, Solomon (2001) suggests that the high divorce rates in shift workers may be due to the strain of being awake at different times from their families. This may not be due to biological factors associated with the circadian rhythm disruption.

• This suggests that it is unwise to assume that the adverse consequences associated with shift work are due to the disruption of biological rhythms.

→ MEDICAL TREATMENT

• Another strength of research into circadian rhythms is that 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. This has led to the field of chronotherapeutics - 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 when taken last thing at night. This is because heart attacks are most likely to occur early in the morning, so timing of drug releases can ensure that the drug is released into the bloodstream during the vulnerable period.

• This has real-word application as it has led to the development of effective drug treatments.

  Conflicting Evidence:

  → INDIVIDUAL DIFFERENCES

• One limitation of research into circadian rhythms is that generalisations are difficult to make.

• Many of the studies used small samples of participants. These may not be representative of the wider population, which limits the extent to which meaningful generalisations can be made.

• Furthermore, research has shown that there are individual differences in circadian rhythms. For example, Czeisler et al (1999) found that individual sleep/wake cycles can vary from 13 to 65 hours.

• Duffy and Czeilser (2002) revealed that some people display a natural preference for going to bed early and rising early (larks) whereas some people prefer to go to bed late and wake up late (owls).

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