Biological Rhythms: Circadian Rhythms

Biological rhythms definition

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

Circadian rhythms definition

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.

Biological Rhythms

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 2 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) and in some cases much longer (circannual rhythms). Circadian rhythms are those 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. - 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 resurfaced in mid-September, 1962, after 2 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 6 months in a Texan cave. - In each case, his 'free running' (biological clock 'left to its own devices') 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 Jurgen Aschoff & Rutger Wever, 1976, who convinced a group of participants to spend 4 weeks in a World War 2 bunker deprived of natural light. - All but 1 of the participants (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24 and 25 hours. - 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 the number of daylight hours, typical mealtimes, etc.).

12 people live in a cave for 3 weeks, not knowing that the clocks were sped up to 22 hrs. Only one person was able to comfortably adjust.

Other research 2

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 3 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 speeded up the clock (unbeknown to the participants) so an apparent 24-hour day eventually lasted only 22 hours. - It was revealed that only 1 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 1

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 3x 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.

Circadian Rhythm - Evaluation: Shift Work (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 2

Another strength of research into circadian rhythms is that it has been used to improve medical conditions. 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 a 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.

Weakness 1

One limitation of research into circadian rhythms is that generalisations are difficult to make. Aschoff and Wever, and Siffre studies are based on very small samples of participants (just 1 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.

Strength 3

A number of researchers (e.g. Wolfson and Carskadon, 1998) recommend that the school day start a couple of hours later to fit in with the typical teenage chronotype (sleep pattern). Hormonal shifts in the teenage body mean that getting to sleep becomes more difficult and therefore adolescent students tend to be rather sleepy at the start of the school day. Research has shown benefits for academic and behavioural performance when lessons start later in the day, including reduced dependence on caffeine (Adolescent Sleep Working Group 2014). However, shifting the start of the day is disruptive for parents and teachers, and it limits the number of extracurricular activities after school. Also, critics of the proposal suggest that a later school day would not actually reduce sleep deprivation, it would simply mean that teenagers would stay up later and still be exhausted!