Biological Rhythms: The Circadian Rhythm

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Circadian Rhythms

  • Rhythms that last about 24 hours; two best known are the sleep-wake cycle and the body temperature cycle.
  • The Sleep-Wake Cycle
  • Your sleep-wake cycle is governed by knowing what time of the day it is; these are governed by external cues which are important in guiding the cycle.
  • There is also an endogenous clock which is free running; it sets a cycle for 24-25 hours.
  • Under normal cycles, the endogenous clock does not work alone; the external cues help adjust it to your envornment.
  • The circadian rhythms persist despite isolation; which demonstrates the existence of an endogenous clock.
  • Yet studies also reveal that external cues are important because the clock was not accurate.
  • Core Body Temperature
  • This is the best indicator of the circadian rhythm. Lowest at 4:30am and highest at 6:00pm
  • Temeprature dip is a bi-daily rhythm, which is an example of ultradian rhythms.
  • Hormones
  • Hormone production also follows a circadian rhythm.
  • Cortisol is at its lowest around midnight and peaks around 6am.
  • It is a hormone produced when we are stressed but it is also related to making us alert when we wake up. 
  • Melatonin and growth hormone are two others which have a clear circadian rhythm, which both peak around midnight.

Evaluation

  • Research Support for a Free-Running Rhythms
  • Michael Siffre was a French cave explorer who specialised in studying the endogenous clock.
  • He spent long periods of time living underground to study his own biological rhyth,s - he had no external cues.
  • Only thing influencing his behaviour was his endogenous clock.
  • After his first underground stay of 61 days, he resurfaced

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