circadian rhythms Flashcards
Biological rhythms
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. 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 (see page 50).
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).
Circadian rhythms are those rhythms that last for around 24 hours (circa is Latin for ‘about’ and diem for day). Two examples of circadian rhythms are the sleep/wake cycle and core body temperature
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.
However the sleep/wake cycle is also governed by an internal (endogenous) pacemaker - a biological lock’ 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 if the biological clock was left to its own devices’ without the influence of external stimuli such as light (what researchers refer to as free-running’)? If we had no idea whether it was night or day would we still fall asleep and wake up at regular times? Researchers have tried to answer this question.
Siffres cave study
Michel Siffre (pronounce ‘Seef”) is a self-styled caveman who has spent several extended periods underground to study the effects on his own biological rhythms. Deprived of exposure to natural light and sound, but with access to adequate food and drink, Siffre resurfaced in mid-September 1962 after two months in the caves of the Southern Alps believing it to be mid-August! A decade later he performed a similar feat but this time for six months in a Texan cave.
In each case, his ‘free-running’ biological rhythm settled down to one that was just beyond the usual 24 hours (around 25 hours) though he did continue to fall asleep and wake up on a regular schedule.
Other research
Similar results were recorded by Jürgen Aschoff and Rütger Wever (1976) who convinced a group of participants to spend four weeks in a World War 2 bunker deprived of natural light. All but one of the participants (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24 and 25 hours. Both Siffres 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 the number of daylight hours, typical mealtimes, etc.).
Despite this, we should not overestimate the influence of exogenous zeitgebers on our internal biological clock. Simon Folkard et al. (1985) studied a group of 12 people who agreed to live in a dark cave for three weeks, retiring to bed when the clock said 11.45 pm and rising when it said 7.45 am. Over the course of the study, the researchers gradually speeded up the clock (unbeknown to the participants) so an apparent 24-hour day eventually lasted only 22 hours!
It was revealed that only one of the participants was able to comfortably adjust to the new regime. This would suggest the existence of a strong free-running circadian rhythm that cannot easily be overridden by exogenous zeitgebers.
Strength- 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 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).
This shows that research into the sleep/wake cycle may have real-world economic
implications in terms of how best to manage worker productivity.
Counterpoint 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 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.
This suggests that it may not be biological factors that create the adverse consequences associated with shift work.
Strength-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. 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 (e.g. Bonten et al. 2015).
This shows that circadian rhythm research can help increase the effectiveness of
drug treatments.
Limitation-individual differences
One limitation of research into circadian rhythms is that generalisations are difficult to make.
The studies described on the facing page (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 Charles Czeisler et al. (1999) found individual differences in sleep/wake cycles varying from 13 to 65 hours. In addition, a study by Jeanne 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.
This means that it is difficult to use the research data to discuss anything more than
averages, which may be meaningless.
