Circadian rhythms

• What are biological rhythms? What are endogenous pacemakers and exogenous zeitgebers?

  • Distinct patterns in the body's activity corresponding to a timely cycle
  • Endogenous pacemakers - our internal biological clocks that regulate biological rhythms
  • Exogenous zeitgebers - external environmental cues that influence and reset our internal biological clock
• What are circadian rhythms?

  • Biological rhythms that last for 24 hours like the sleep/wake cycle and body temperature
• What is the sleep/wake cycle?

  • The recurring pattern of transitions between sleep and wakefulness, subject to 24 hours
  • Governed by an endogenous pacemaker called the suprachiasmatic nucleus - lying above the optic chiasm that provides information from the eyes about light levels
  • Also affected by exogenous zeitgebers like light that can reset the SCN and affect whether we feel tired or not
• What is free-running?

  • A biological rhythm purely based on endogenous pacemakers without the influence of exogenous zeitgebers
• What was Siffre's cave study?

  • Siffre (1962) studies his own circadian rhythms by living in a cave, deprived of exposure to natural light and sound for 2 months and then for 6 months a decade later
  • In both instances his free running circadian rhythm was 25 hours
• What other research is there into circadian rhythms?

  • Aschoff & Rutger (1976): placed participants in a WW2 bunker for 4 weeks depriving them of natural light - all but 1 ppts. displayed a circadian rhythm between 24 and 25 hours, suggesting the natural sleep/wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with our 24-hour day like mealtimes
  • Folkard et al. (1985): 12 participants in a dark cave were isolated from natural light for 3 weeks - over the course of the study researchers sped up the clock so the average day lasted 22 hours
  • Most participants could not comfortably adjust to the new clock showing there is a strong free-running circadian rhythm influenced more strongly by the internal body clock than environment
• What other endogenous pacemakers are there?

  • Core body temperature - sleep occurs when this drops + rises during the last hours of sleep which prompts feelings of alertness in the morning, drops at 2-4pm can explain afternoon sleepiness
  • Hormone production - the pineal gland releases high levels of melatonin during dark hours which encourages sleep, and the presence of light makes melatonin drop which leads to us waking up
• What is one strength of research into circadian rhythms?

  • Contribution to shift work: provides an understanding of the adverse consequences that occur when circadian rhythms are disrupted
  • Boivin et al. (1996) found that night workers engaged in shift work experience a period of reduced concentration around 6 in the morning, meaning they are more likely to make mistakes + have accidents
  • Knutsson (2003) found that shift workers are 3x more likely to develop heart disease than people who work typical work patterns
  • Means research can have real-world economic implications in terms of how to best manage worker productivity
• What is another strength of research into circadian rhythms?

  • Improving medical treatments: circadian rhythms co-ordinate a number of basic processes such as heart rate and digestion that rise and fall during the course of a day
  • Chronotherapeutics - how medical treatment can be administered in a way that corresponds to biological rhythms
  • Bonten et al. (2015): aspirin reduces blood platelet activity -> reduces risk of heart attack -> heart attacks more likely to occur in morning -> aspirin should be effective if taken last thing at night
  • Shows circadian rhythm research can help increase the effectiveness of drug treatments
• What is one limitation of research into circadian rhythms?

  • Poor control in supporting studies: in early research studies, participants were isolated from natural light but not artificial light as they thought it would not affect circadian rhythms
  • Czeisler et al found that participants rhythms could be altered between 22-28 hours using dim artificial lighting
  • Shows early studies establish poor cause-and-effect due to serious extraneous variables and therefore cannot evidently show that circadian rhythms are more strongly influenced by endogenous pacemakers
• What is another limitation of research into circadian rhythms?

  • Individual differences: research studies are based on very small participant samples making it difficult to generalise, other research shows that sleep/wake cycles may vary widely
  • Czeisler et al. (1999) found individual differences in S/W cycles varying from 13-65 hours
  • Duffy et al. (2001) found that some people have a natural preference for going to bed early and waking up early (larks) and others prefer the opposite (night owls)
  • Means it is difficult to use the research data to make generalisations about everyone's sleep/wake cycles