Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous Pacemakers

  • internal body clocks that regulate biological rhythms
• Exogenous Zeitgebers

  • external cues that may affect our biological rhythms, such as the influence of light on the wake-sleep cycle
• The main Endogenous Pacemakers - SCN

  • the SCN receives information about light levels via the optic nerve, even when our eyes are closed
  • the SCN also regulates the secretion of melatonin in the pineal gland
• Exogenous Zeitgebers - light

  • the biological clock or the SCN is reset each day by sunrise and sunset
  • this process is called entrainment
  • therefore, sleeping and wakefulness are determined by an interaction of internal and external factors
• The pineal gland

  • governed by the light
  • secretes melatonin
  • melatonin secretion is highest in the middle of the night
  • the purpose of melatonin is to make you feel sleepy
• Social Cues as Exogenous Zeitgebers

  • Aschoff (1971) - individuals are able to compensate for the absence of zeitgebers natural light by responding to social zeitgebers
  • a study found that circadian rhythms of air travelers adjusted more quickly if they went outside more at their destinations (exposed to social cues)
• DeCoursey et al (2000) - evaluation

  • destroyed the SCn connections in the brains of 30 chipmucks who were then returned to their natural habitat and observed for 80 days
  • the sleep wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators
  • presumably because they were awake and vulnerable to attack when they should have been asleep
  • not generalisable
  • unethical
• Touitou (2017) - evaluation

  • exposure to artificial light at night results in a disruption of the circadian system, which has adverse effects on health
  • teenagers spend increasing amounts of time on electronic media at night
  • the LED bulbs of these devices are enriched with a blue light component which can disrupt the circadian clock, which leads to the suppression of melatonin secretion and circadian disruptions
  • as a result, adolescents become irregular, shortened and delayed
• Vetter et al (2011)

  • demonstrated the importance of light in the regulation of the sleep wake patterns of 2 groups
  • one group remained in a warm artificial light over 5 weeks and the other group experience blue enriched light
  • over the course of the study, sunrise advanced by 42 minutes
  • those in the blue light group did not show the same 42 minute adjustment and instead synchronised their rhythms to office hours
  • those in the warm light were able to synchronise their circadian rhythms each day with natural light at dawn
• Circadian Rhythms

  • cyclical changes in the body's activity
  • circadian rhythms are a type of biological rhythm where a pattern of behaviour occurs approximately every 24 hours
  • it is set and resent by environmental light levels
  • for example, the sleep wake cycle, body temperature and hormone production
  • all rhythms are governed by the body's internal biological clocks (endogenous pacemakers) and external changes in the environment (exogenous zeitgebers)
• The sleep/wake cycle

  • feeling drowsy at night and alert during the day is the effect of daylight
  • daylight is an important zeitgeber on our sleep/wake cycle
• Siffre's cave study

  • Siffre is a self-styled cave man who has spent extended periods of time underground
  • he spent 2 months in 1962 in the caves of the Southern Alps with access to adequate food and drink
  • he did this again for 6 months in a Texan cave 10 months later
  • his free-running biological rhythm settled down to one that was just beyond the usual 24 hours, though he did continue to fall asleep and wake up on a regular schedule on both occasions
• Evaluation of Circadian Rhythms

  • given researchers a better understanding of the adverse consequences that can occur as a result of their disruption (Boivin et al 1996)
  • research has also suggested a relationship between shift work and poor health, shift workers are three times more likely to develop heart disease which may be in part due to the stress of adjusting to different sleep/wake patterns
  • research shows that there are certain peak times during the day or night when drugs are likely to be at their most effective
  • people involved in case studies may not be representative of the wider population and this limits the extent of meaningful generalisations