Circadian Rhythms

• Biological rhythms are influenced by internal body clocks (endogenous pacemakers) and external changes to the environment (exogenous zeitgebers)
• Circadian rhythms are a type of biological rhythm that is controlled by the 24-hour cycle of light and dark.
• Circadian rhythms regulate the sleep/wake cycle and core body temperature
• The purpose of circadian rhythms is to optimise an organism's physiology and behaviour in order to meet the demands of the day/night cycle
• circadian rhythms are synchronised by the suprachiasmatic nucleus in the hypothalamus
• Suprachiasmatic nucleus gets reset through light. This is called entrainment
• lightness and darkness are external clues which determine when humans and animals should be asleep and awake
• Siffre's cave study aimed to investigate the influence of the absence of external cues on the circadian rhythms
• Siffre spent 61 days in a cave sleeping and eating when it felt appropriate, allowing his internal body clock to govern his behaviour
• Siffre found that his biological rhythm was 25 hours long
• Siffre maintained a regular sleep-wake pattern
• Siffre concluded that the biological clock ran at a natural rate and circadian rhythms persist despite the absence of exogenous zeitgebers. However, we need entrainment to keep a 24 hour cycle
• Aschoff and Wever (1976) placed participants in a WW2 bunker for 4 weeks where there was no natural light
• Aschoff and Wever found that participants had circadian rhythms of over 24 hours and that our natural sleep/wake cycle is entrained by exogenous zeitgebers
• Folkard et al (1985) had 12 people stay in a cave for 3 weeks, sleeping and waking at a set time (governed by a clock)
• Folkard et al sped up the clock as the study went on so that a 'day' only lasted 22 hours
• Folkard et al found that only one participant adjusted to this 22 hour day. They concluded that the free running biological clock is able to override some external changes in environment
• core body temperature is also governed by circadian rhythms
• body temperature affects our mental abilities, the warmer we are the better our cognitive performance
• sleep occurs when body temperature decreases, brain waves slow down, and breathing rate decreases
• we wake up when our body temperature begins to rise
• hormone release follows a circadian rhythm
• the production and release of melatonin from the pineal gland has peak levels during hours of darkness
• STRENGTHS

  • practical application to shift work
  • practical application to drug treatment
• LIMITATIONS

  • use of case studies and small samples
  • poor control in studies