Biological Rhythms: Circadian Rhythms Flashcards
(13 cards)
What are biological rhythms and what do they influence?
All living organisms - plants, animals and people - are subject to biological rhythms and these exert an important influence on the way in which body systems behave.
What governs all biological rhythms?
All biological rhythms are governed by two things - the body’s internal biological clocks, which are called endogenous pacemakers and external changes in the environment known as exogenous zeitgebers.
What are the different types of biological rhythms?
Some of these rhythms occur many times during the day (ultradian rhythms). Others take longer than a day to complete (infradian rhythms) and in some cases much longer (circannual rhythms).
What are circadian rhythms?
Circadian rhythms are those rhythms that last for around 24 hours.
How does daylight affect the sleep/wake cycle?
The fact that we feel drowsy when it’s night-time and alert during the day demonstrates the effect of daylight - an important exogenous zeitgeber - on our sleep/wake cycle.
What is the SCN and how does light affect it?
The sleep/wake cycle is also governed by an internal (endogenous) pacemaker - a biological ‘clock’ called the suprachiasmatic nucleus (SCN). The SCN lies just above the optic chiasm which provides information from the eye about light. Exogenous zeitgebers (light) can reset the SCN.
What did Siffre’s cave studies show about circadian rhythms?
Michel Siffre is a self-styled caveman who spent extended periods underground to study the effects on his own biological rhythms. Deprived of natural light and sound, with food and drink, he resurfaced in mid-September 1962 after two months in the Southern Alps believing it to be mid-August. A decade later he repeated this for six months in a Texan cave. In both cases, his free-running rhythm settled to just beyond 24 hours (around 25 hours), though he continued to sleep and wake regularly.
What did Aschoff and Wever’s study show about circadian rhythms?
Jürgen Aschoff and Rütger Wever (1976) had participants spend four weeks in a World War 2 bunker without natural light. All but one showed circadian rhythms between 24 and 25 hours. This and Siffre’s findings suggest our natural cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers like daylight and mealtimes.
What did Folkard et al. (1985) show about the power of endogenous pacemakers?
Simon Folkard et al. (1985) studied 12 people in a dark cave for three weeks. They were told to sleep from 11:45 pm to 7:45 am. The researchers sped up the clock so that a 24-hour day became 22 hours. Only one participant could adjust, suggesting a strong free-running circadian rhythm that cannot easily be overridden by exogenous zeitgebers.
PEEL: Strength - Understanding consequences of disruption
Point: One strength of research into circadian rhythms is that it provides an understanding of the adverse consequences that occur when they are disrupted (desynchronisation). Evidence: 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 pointed to a relationship between shift work and poor health - shift workers are three times more likely to develop heart disease than people who work more typical work patterns (Knutsson 2003). Explain: This shows that research into the sleep/wake cycle may have real-world economic implications in terms of how best to manage worker productivity. Link: This means research into biological rhythms has practical value for improving occupational health and safety.
PEEL: Limitation - Correlational methods in shift work
Point: However, studies investigating the effects of shift work tend to use correlational methods. Evidence: 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, Charlene Solomon (1993) 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. Explain: This suggests that it may not be biological factors that create the adverse consequences associated with shift work. Link: Therefore, conclusions about cause and effect in shift work studies should be drawn with caution.
PEEL: Strength - Medical treatment application
Point: Another strength of research into circadian rhythms is that it has been used to improve medical treatments. Evidence: 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 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 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. Research has supported this (eg. Bonten et al. 2015). Explain: This shows that circadian rhythm research can help increase the effectiveness of drug treatments. Link: Therefore, it has significant practical applications in medical care.
PEEL: Limitation - Individual differences
Point: One limitation of research into circadian rhythms is that generalisations are difficult to make. Evidence: The studies described (Aschoff and Wever, and Siffre) are based on very small samples (just one in the case of Siffre). Sleep/wake cycles may vary widely. Charles Czeisler et al. (1999) found sleep/wake cycles varied from 13 to 65 hours. Jeanne Duffy et al. (2001) found some are ‘larks’ (early risers) and others ‘owls’ (late risers). Even Siffre found his rhythm slowed as he aged. Explain: This means that it is difficult to use the research data to discuss anything more than averages, which may be meaningless. Link: As a result, findings from circadian rhythm research may lack generalisability.
