6. biological rhythms - circadian Flashcards
All living organisms 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.
Some of these rhythms occur many times during the day
ultradian rhythms.
Others take longer than a day to complete
infradian rhythms.
Circadian rhythms
rhythms that last for around 24 hours.
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 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.
SIFFRE’S CAVE STUDY
Michel Siffre 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 found his 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
Despite this, we should not overestimate the influence of exogenous zeitgebers on our internal biological clock. Folkard et al. 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.
CORE BODY TEMPERATURE
Varies by about 2°C a day.
Lowest at around 4 am at 36 and peaks at 6pm at 38.
Evidence suggests the warmer we are internally, the better our cognitive performance:
Folkard
Folkard demonstrated how children who had stories read to them at 3pm showed superior recall and comprehension after compared to children who had the same stories read to them at 9am.
AO3: strength of circadian rhytms
practical application - 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.
For example, night workers engaged in shift work experience a period of reduced concentration around 6 in the morning meaning mistakes and accidents are 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 people who work more typical work patterns.
This shows that research into the sleep/wake cycle may have real-world economic implications in terms of how best to manage worker productivity.
AO3: strength of circadian rhytms
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 chrono therapeutics - 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.
This shows that circadian rhythm research can help increase the effectiveness of drug treatments.
AO3: limitation of circadian rhytms
individiual differences - Czeisler, Siffre
One limitation of research into circadian rhythms is that generalisations are difficult to make.
The cave studies 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. 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.
