Biological Rhythms: Circadian Rhythms

• Define 'biological rhythm'


  Distinct patterns of changes in body activity that conform to cyclical time periods. Biological rhythms are influenced by internal body clocks (endogenous pace makers) and external changes to the environment (exogenous zeitgebers).
• Define 'circadian rhythm'

  Biological rhythms subject to a 24 hours cycle, which regulate a number of bodily processes such as the sleep/wake cycle and changes its 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 affect of daylight - an important exogenous zeitgber - on our sleep/wake cycle.

  - The sleep/wake cycle is also governed by an endogenous pacemaker - a biological clock known as the suprachiasmatic nucleus (SCN). This lies just above the optic chiasm which provides information from the eye about light. Exogenous zeitgebers such as light reset the SCN.
• Siffre's (1975) cave study


  - Siffre tested free-running circadian rhythms by spending 179 days in a cave with no natural light or other potential exogenous zeitgebers such as clocks or social cues.

  - Siffre's body clock (endogenous pacemaker) maintained a regular cycle of around 25 hours without exogenous zeitgebers, longer than the expected 24 hours.

  - This suggests that sleep/wake cycle is free-running but needs entrainment to keep to the 24 hour day/night cycle.
• Other research

  - Aschoff and Wever (1976) - A group of participants spent 4 weeks in a WW2 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 Siffre's and Aschoff and Wever's studies 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 24hr day, e.g. daylight hours, typical mealtimes etc.

  - 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 7:45pm. Over the course of the study, the researchers gradually sped up the clock (unbeknown to the participants) so an apparent 24 hour day only lasted 22 hours. It was revealed that only one of the participants was able to comfortably adjust to this new regime. This would suggest the existence of a strong free-running circadian rhythm that cannot easily be overriden by exogenous zeitgebers.
• Evaluating circadian rhythms: Shift work (with counterpoint)

  - A strength of research into circadian rhythms is that it provides an understanding of the adverse consequences that occur when they are disrupted (desynchronisation).

  - Boivin et al. (1996) - night workers engaged in shift work experience a period of reduced concentration around 6am (a circadian trough) meaning mistakes and accidents are more likely.

  - Knutsson (2003) have 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.

  - 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.

  - Solomon (1993) concluded that high divorce rates in shift workers may 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.
• Evaluating circadian rhythms: Medical treatment

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

  - Circadian rhythms coordinate a number of the body's basic processes such as heart rate, digestion and hormone levels. These rise and fall during the course of the day which has led to the emergence of the field of chronotherapeutics - how medical treatment can be administered in a way that corresponds to a person's biological rhythms.

  - For example, aspirin can be used as a treatment for heart attacks most effectively 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 from Bonten et al. (2015) has supported this.

  - This shows that circadian rhythm research can help increase the effectiveness of drug treatments.
• Evaluating circadian rhythms: Individual differences

  - A limitation of research into circadian rhythms is that generalisations are difficult to make.

  - Aschoff and Wever's and Siffre's studies are based on very small samples. It seems that sleep/wake cycles may vary widely from person to person.

  - Czeisler et al. (1999) found individual differences in sleep/wake cycles varying from 13 to 65 hours.

  - A study be Duffy et al. (2001) revealed that some people have a natural preference for going to bed early and rising early (aka larks) whereas others prefer the opposite (owls).

  - Even Siffre in a later study (1999) recognised that his 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.
• Evaluating circadian rhythms: Shifting the school day

  - A number of researchers such as Wolfson and Carskadon (1998) recommend that the school day starts a few hours later to fit in with the typical teenage 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 school day is disruptive for parents and teachers, and it limits the number of extracurricular activities after school.

  - Critics also say that a later school day wouldn't reduce sleep deprivation as it would simply mean that teenagers stay up later and still be exhausted.