Circadian rhythms: biological

• Biological rhythms


  Distinct patterns of changes in body activity that conform to cyclical changes in the environment
• All living organisms are subject to biological rhythms
• Biological rhythms are governed by

  • Endogenous pacemakers - the body's internal biological 'clocks'
  • Exogenous zeitgebers - external changes to the environment
• Types of biological rhythms

  • Ultradian rhythms - rhythms that take part at any time during the day
  • Infradian rhythms - takes longer than a day to complete
  • Circannual rhythms - takes much longer
  • Circadian rhythms - those rhythms that last for around 24 hours
• Circadian rhythms


  The sleep/wake cycle
• Daylight


  Important exogenous zeitgeber that effects our sleep/wake cycle
• Siffre's cave study


  • Michel Siffre is a cave man who spent long periods of time underground to study the effects on his own biological rhythms
  • Deprived of exposure to natural light and sound, but had access to food and drink
  • Re-surfaced after 2 months in the cave, believing it was mid August when it was actually mid September
  • His 'free running' biological rhythm settled down to one that was just beyond the usual 24 hours (25 hours)
  • He continued to fall asleep and wake up on a regular schedule
• Jürgen Aschoff and Rütger Wever study

  • Convinced a group of participants to spend four weeks in WWII, deprived from natural light
  • All but 1 participant displayed a circadian rhythm between 24 and 25 hours
• Both Siffre's and Aschoff and Wever's experiences suggest that natural sleep/wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with our 24-hour-day
• Simon Folkard et al study


  • 12 people who agreed to live in the dark cave for 3 weeks, going to bed when the clock said 11.45 and rising when it said 7.45 am
  • The researchers slowly started speeding the clock down without the participants knowing, eventually the day only lasted 22 hours
  • Only 1 participant was able to comfortably adjust to the new regime, suggesting the existence of a strong free-running circadian rhythm that cannot be easily overridden by changes in the external environment