circadian rhythms

• A biological rhythm is a distinct pattern of changes in body activity that conform to cyclical time periods
• Biological rhythms are influenced by internal body clocks (endogenous pacemakers) and external changes to the environment (exogenous zeitgebers)
• Circadian rhythm - a type of biological rhythm, subject to a 24 hour cycle, which regulates a number of body processes
• examples of circadian rhythms:

  • The sleep/wake cycle
  • Heart rate
  • Metabolic rate
  • Breathing rate
  • Body temperature
• all circadian rhythms reach maximum values in late afternoon and minimum values in early hours of the morning
• French cave explorer, Siffre, lived for 6 months in a cave with no natural light or anything to tell him the time
• Siffre's biological clock was allowed to free-run and he was wired up so some body functions could be recorded
• At first, Siffre's sleep schedule was erratic but it settled to an average pattern of 25 hours instead of 24. This shows he had some internal biological mechanism which regulated the cycle even in absence of environmental cues
• Aschoff and Wever (1976) convinced a group of pps to spend 4 weeks in a WW2 bunker deprived of natural light. All but one pps displayed a circadian rhythm between 24 and 25 hours
• Folkard et al (1985) has a sample of 12 people live in a dark cage for 3 weeks, retiring to bed when the clock said 11:45 and rising when it said 7:45am. The researchers gradually sped up the clock (unknown to pps) so an apparent 24 hour day only lasted 22.
• Not one of the pps in Folkard's study were able to comfortably adjust to the new routine. Suggests the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment
• Knowledge of circadian rhythms provides researchers with a better understanding of the consequences of disruption to sleep
• Bovin found that night shift workers may experience lapse in concentration, poorer health possibly linked to stress of adjusting to different sleep patterns
• research into circadian rhythms is important as it may have economic implications in terms of how best to manage worker productivity
• Circadian rhythms coordinate a number of the body’s basic processes such as heart rate, digestion and hormone levels, which has an effect on pharmacokinetics, which is the action of drugs on the body and how well they are absorbed/distributed
• Research into circadian rhythms has revealed that there are peak times during the day or night when drugs are at their most effective
• Baraldo reported that research into circadian rhythms has led to the development of guidelines for the administration of drugs for conditions such as cancer, cardiovascular, respiratory and epilepsy. This has led to improved treatments for patients and increased the effectiveness of this medication
• Studies into the sleep/wake cycle have involved small groups of pps or in the Siffre’s case study, one individual and so there are issues with generalisability. Individual differences e.g the speed at which individual biological clocks run could influence the findings
• Although pps in these studies were deprived of natural light, they had access to artificial light
• Czeisler et al (1999) were able to adjust pps circadian rhythms from 22 to 28 hours using dim lighting
• Czeisler et al found that individual sleep cycles can vary between 13 to 65 hours
• Duffy et al (2000) also found that some people had a natural preference to go to bed early and get up early whilst others preferred going to bed late and getting up late