Circadian Rhythms

• Circadian rhythms occur over a 24 hour period.
• The sleep - wake cycle is a circadian rhythms and is governed by internal and external mechanisms

  • endogenous pacemakers - a biological clock left to its own devices without the influence of external stimuli (light) is free-running
  • exogenous zeitgebers - the fact we feel drowsy at night and alert during the day shows the effect of daylight.

  The basic rhythms is governed by the superchiasmatic nucleus (SCN) which is situated above the optic chiasm which provides information about light from the eyes. Exogenous zeitgebers can reset the SCN.
• Siffre Cave Study

  Sifree spent long periods of time in dark caves to examine the effects of free running biological rhythms. 2 months in 1962 and 6 months in 1979s. In each, Siffre’s free running circadian rhythm settled down to 25 hours and he did have a regular sleep wake cycle.
• Aschoff and Wever WW2 Bunker Study

  A group of participants spent 4 weeks in a WW2 bunker deprived of natural light. All but one (whose s/w cycle extended to 29 hours) displayed a circadian rhythm of 24/25 hours.
• Siffre’s experience and the Bunker Study suggest the natural sleep - wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with our 24 hour day - daylight hours, mealtimes.
• Folkard’s Cave Study

  Folkard studied a group of 12 people who lived in a dark cave for 3 weeks, going to be when the clock said 11:45pm and waking when it said 7:45am. The researchers gradually sped up the clock (unbeknown to the participants) to a 22 hour day. Only one participant comfortable adjusted to the new regime. This suggests the existence of a strong free running circadian rhythm isn’t controlled by exogenous zeitgebers.