Sleep-Wake cycle

• Endogenous pacemakers are internal body clocks that keep biological processes to time.
• Exogenous zeitgebers are external cues that entrain internal body clocks, altering it to match the environment.
• Circadian rhythms are the biological rhythms that lasts around 24 hours. This regulates:

  • Sleep and wake,
  • Release of hormones,
  • Varying blood pressure and body temperature.
• Endogenous pacemakers will be entrained by the exogenous zeitgebers to a new time zone, but this make take several days (resulting in jet lag).
• The sleep-wake cycle is the pattern of sleep and wakefulness that repeats every 24 hours. There must be an internal body clock (endogenous pacemaker) that keeps this pattern.
• Suprachiasmatic nucleus (SCN):

  • The endogenous pacemaker of the sleep wake cycle.
  • Known as the master clock.
  • Part of the hypothalamus within the limbic system.
  • Located at the optic chiasm (point of optic nerves where they cross).
  • When light is detected, it sends the light to the pineal glad to stop the production of melatonin.
• Siffre (1975) - Free running circadian rhythms:

  • Spent 6 months in a cave with no natural light or other potential EZ (like clocks or social cues).
  • His body clock maintained a regular cycle of around 25 hours without EZ, longer than the expected 24 hour cycle.
  • This suggests the EP for the sleep wake cycle is free running but needs entertainment to keep the the 24 hour day-night cycle.
• Criticisms of natural light isolation studies:

  • Psychologists suggest artificial lights disrupted the measurements of the free running circadian rhythm by acting as an EZ.
  • In a study controlling artificial light, it was found the circadian rhythm was closer to 24 hours with little drift.
• Evidence for light as an exogenous zeitgeber:

  • Vetter conducted a study with 27 office workers. They were exposed to strong blue light, shifting their rhythms to match the office lighting.
  • A control group of another 27 workers with normal office lighting matched the shifting time of the natural light of dawn.
  • This suggests light is an EZ and artificial lights can disrupt the EP by overriding natural lights as the primary EZ.
• Evidence of the SCN being the EP:

  • Morgan conducted a study with golden hamsters (24 hour circadian rhythm) and Tau mutant hamsters (20 hour circadian rhythms).
  • He transplanted the brains from the normal hamsters into the mutant hamsters and found they had then gained a sleep wake cycle of 24 hours.
  • This suggests the SCN is the main EP in the sleep wake cycle.
• Speeding up the sleep-wake cycle - Folkard (1985):

  • Studied 12 participants living in a cave for 3 weeks. Participants went to bed when the clock said 11.45pm and woke up at 7.45am.
  • Folkard sped up the clocks making the cycle 22 hours without telling the participants.
  • Only one participant was able to adjust to the new regime.
  • This suggests there is a strong circadian rhythm that cannot easily be overridden by changes in the external environment.
• Strengths of the circadian rhythm:

  • Changes in night shift work - Night shifts were found to lead to desynchronisation of circadian rhythms and can lead to adverse effects. Boivin found workers experience reduced concentration around 6am, where mistakes and accidents occurred more.
  • Knutsson found they are also 3 times more likely to develop heart disease due to stress of adjusting to sleep/wake cycles and lack of poor quality sleep during the day.
  • This research has many economic implications, in terms of maintaining worker productivity and preventing accidents in the work place.
• Strengths of circadian rhythms:

  • Practical application - Understanding of circadian rhythms and their impact on health can help determine the best time to administer drug treatments. Research has revealed that there are certain peak times during the day or night when drugs are likely to be at their most effective.
  • This led to development of guidelines to do with timing of drug dosing for a range of medications including anticancer, cardiovascular, respiratory, anti-ulcer and anti-epileptic drugs.
• Limitations of circadian rhythms:

  • Generalisability - Studies of the sleep/wake cycle tend to involve small groups of participants (eg - Aschoff and Wever) or studies of single individuals (eg Siffre). The people involved may not be representative of the wider population, limiting the extent to which meaningful generalisations can be made.
• Limitations of the circadian rhythms:

  • Lack of control over variables - In cave studies, exposure to artificial light was not controlled. It was assumed it was only natural light that has an influence on our biological rhythms, however Czeisler found that dim artificial lighting could adjust the circadian rhythm between 22-28 hours. This suggests such studies may lack internal validity.