Biological rhythms: Circadian rhythms:

  • Circadian rhythms:
  • AO1:
  • Biological rhythms are perodic activity, governed by: Internal body 'clocks' (endogenous pacemakers) or external changes in the environment (exogenous zeitgeibers). Some of these rhythms occur many times a day (ultadian rhythms); others take less than a day to complete (infradian rhythms) and in some cases, much longer (circannual rhythms). 
  • The circadian rhythm lasts for about 24 hours. Circa meaning 'about' and diem meaning 'day'. There are several important types of circadian rhythm such as the sleep/wake cycle. 
  • The sleep/wake cycle is governed by internal and external mechanisms:
  • Exogenous zeitgibers - the fact we feel drowsy when its night-time and altert during the day shws the effect of daylight. Edogenous pacemakers - a biological clock 'left to its own devices' without the influence of external stimuli (light) is called 'free-running'. There is basic rhythm governed by the superchiasmatic nucleus (SCN), which lies just above the optic chiasm and recieves information about light directly from this structure. The exogenous zeitgeber (light) can reset the SCN. 
  • Study:
  • French caver Siffre spent long periods of time in dark caves to examine the effects of free-running biological rhythms - 2 months in the caves of the Southern Alps and 6 months in a Texan cave. In each case study, Siffre's free-running circadian rhythm settled down to just above usual 24 hours (about 25 hours). Importantly, he did have a regualr sleep/wake cycle. 
  • a group of participants spent 4 weeks in a WW2 bunker deprived from any source of natural light (Aschoff and Wever). All but one (whose sleep/wake cycle extended to 29 hours) displayed. circadian rhythm of around 24-25 hours.
  • Siffre's experience and the bunker study suggest that the 'natural' sleep/wake cycle may be slighy longer than 24 hours


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