Biological rhythms

• Cyclical changes in the way that the body systems behave. These are influenced by mental clocks as well as external changes in the environment.
• Circadian rhythms

  • cycles that last for around 24 hours
  • they optimise an organism's physiology and behaviour to best meet their varying demands of the day/night cycle
  • they are driven by our body clocks (endogenous pacemakers) which are found in all cells of the body and synchronised by the master pacemaker, the SCN, or the suprachiasmatic nuclei, which is found in the hypothalamus
  • this pacemaker is constantly reset so that our body remains in tune with the outside world
  • external cues (exogenous zeitgebers) help our body clocks get set to the right environment
• The sleep/wake cycle

  • the circadian rhythm dictates when we should be sleeping and when we should be awake
  • light and darkness are the external factors that determine this
  • homeostasis also tells us when we need to sleep because of the amount of energy used while away for a long period of time
  • gradually makes us sleepier as time goes on
• The sleep/wake cycle (continued)

  • strongest sleep drive occurs in 2 dips, usually between 2-4am and 1-3pm, and the sleepiness we experience during these dips is less intense if we have had sufficient sleep and more intense when sleep deprived
• A03

  Michael Siffre

  • spent long periods of time living underground to study biological rhythms
  • he had no external cues to guide his rhythms, and woke, ate, slept when he felt it was appropriate to do so, on his own body clock
  • natural circadian rhythm settled to just over 24 hours

  Aschoff and Wever

  • convinced a group of Ps to spend 4 weeks in a WWII bunker and deprived it of natural light
  • all but one Ps displayed a circadian rhythm between 24-25 hours
• BUT both studies suggest that the natural sleep wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers assosciated with our 24 hour day.
• Core body temperature

  • varies by about 2 degrees during the day from its lowest about 4.30am to its highest at 6pm (36-38 degrees)
  • sleep occurs when the core body temperature begins to drop
  • it starts to rise during the last hours of sleep which prompts a feeling of alertness in the morning
  • a small drop in body temperature also occurs during 2-4pm which may explain why people get sleepy mid afternoon
• Hormone production

  • the production and release of melatonin from the pineal gland follows a circadian rhythm
  • melatonin encourages feelings of sleep by activating chemical receptors in the brain
  • in the dark, more is produced and when it is light, the melatonin production stops and the person wakes
• A03 - limitation

  Siffre is an individual case study and so cannot be representative to the wider population.

  ALSO there was poor control in the early research studies of circadian rhythms. Although Ps were deprived of natural light, they still had artificial light and Siffre turned on a lap everytime he woke up and turned it off when he went to sleep. It was assumed that the artificial light would not affect free running body clocks. This suggests it is a confounding variable that may affect findings.
• Research into circadian rhythms has value for drug treatments. Studies reveal there are certain peak times when drugs are likely to be most effective and led to the development of timing guidelines for a range of medications.
• A03

  There are useful insights in dealing with shift work. Knowledge of circadian rhythms has given researchers a better understanding of the adverse effects on their disruption. For e.g. nightworkers who are engaged with shift work at around 6am will mean they have reduced concentration which makes accidents more likely.
• A03

  • Research has highlighted that not everyone shares the same circadian rhythm length
  • Czeisler et al found that the length of an individual's cycles can vary, in some cases, 13-65 hours
• A03

  • Recent research has looked at changing the starting time in schools, pilor studies where the school day started at 10am found a significant improvement in results, a reduction in illness and the general atmosphere was overall better
  • This was based on the idea that teenage circadian rhythms are typically two hours after those of adults which also supports a difference in circadian rhythms