Endegounous Pacemakers And Exogenous

• Exogenous = outside the organism


  •The biological-clock is reset each day by cues in the environment – like the light cues of sunrise and sunset.•This process is called entrainment - the opposite of free-running where the biological clock works free of any exogenous cues, including social cues like clocks.•Therefore, sleeping and wakefulness are determined by an interaction of internal and external factors.
• Zeitgeber

  An external cue that can reset the body's internal biological clock
• Light

  • It is a key zeitgeber in humans
  • It can reset the body's main endogenous pacemaker (the SCN)
  • It plays a role in the maintenance of the sleep-wake cycle
• How light resets the body's internal biological clock

  1. Receptors in the SCN are sensitive to changes in light levels during the day

  2. Receptors receive this information from rod and cone cells in the eye via the optic nerve

  3. SCN uses this information to synchronise the activity of the body's organs and glands
• Light resets the internal biological clock each day, keeping it on a 24-hour cycle
• Light

  Has an indirect influence on key processes in the body that control functions such as hormone secretion (e.g. melatonin and cortisol) and blood circulation
• CAMPBELL & MURPHY (1998)

  •They demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes. Fifteen participants were woken at various times by a light pad shone on the back of their knees. The researchers managed to produce a deviation in the participants’ usual sleep-wake cycle of up to 3 hours in some cases.
• Exposure to bright light prior to an east-west flight

  • Decreased the time needed to readjust to local time on arrival
• Treatments

  1. Continuous bright light

  2. Intermittent bright light

  3. Dim light
• Continuous bright light

  Shifted circadian rhythm by 2.1 hours
• Intermittent bright light

  Shifted circadian rhythm by 1.5 hours
• Dim light

  Shifted circadian rhythm by 0.6 hours
• Participants in continuous bright light treatment

  Felt sleepier 2 hours earlier in the evening, and woke 2 hours earlier in the morning
• Social stimuli

  • Mealtimes
  • Social activities
• Zeitgebers

  Environmental cues that synchronize circadian rhythms
• Development of sleep-wake cycle in human infants

  1. Initial cycle is random

  2. Circadian rhythms begin at 6 weeks

  3. Entrained by 16 weeks
• Schedules imposed by parents

  Adult-determined mealtimes and bedtimes
• Adapting to local times for eating and sleeping

  Effective way of entraining circadian rhythms and beating jet lag when travelling long distances
• Research Evidence

  •DeCoursey et al., Ralph et al., Campbell and Murphy and Burgess et al. can all be used for evaluation as supporting evidence.••The animal studies emphasise the role of the SCN in establishing and maintaining the circadian sleep-wake cycle.
• Generalisability

  •There is a problem in generalising the results of animal studies of the sleep-wake cycle to humans. Additionally, particularly in DeCoursey et al.’s study, the ethics issues of the study are subject to criticism. The animals were exposed to considerable harm, and subsequent risk when returned to their natural habitat.••We cannot be certain that the results from animals are completely externally valid and so from the research, we can’t strongly support the role of the SCN as an endogenous pacemaker.
• lack of research replication and internal validity

  •The findings from the Campbell and Murphy study have yet to be replicated. Other psychologists have also been critical of the manner in which the study was conducted. They have suggested that there may have been some limited light exposure to the participants’ eyes which acted as a major confounding variable.••This suggests that the results may not be internally valid and as such, we cannot strongly support the role of light as an exogenous zeitgeber via the skin from the research.
• Real World Application

  •Using our knowledge of the importance of light as an exogenous zeitgeber for the sleep-wake cycle, we can use light exposure to avoid jet lag. Burgess et al. then supports this.••This suggests that the research is externally valid and has potentially important implications for society.
• Isolation Studies Are Rare

  •Only in exceptional circumstances are endogenous pacemakers free-running and unaffected by the influence of exogenous zeitgebers. Total isolation studies like Siffre’s are extremely rare and could be judged as lacking validity for this reason.••In real-life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research.
• •Endogenous pacemakers = internal body clocks that regulate many of our biological rhythms, such as the influence of the SCN and pineal gland on the sleep-wake cycle.
• •Exogenous zeitgebers = external cues that may affect (or entrain or regulate) our biological rhythms, such as the influence of light on the sleep-wake cycle.