exogenous zeitgebers and the sleep/wake cycle

• External environmental factors that reset biological clocks

  • The German word zeitgeber means 'time giver'
  • Resetting biological clocks is a process known as entrainment
  • Without external cues, the free-running biological clock continues to 'tick' in a cyclical pattern
  • Zeitgebers reset the sleep/wake cycle (interaction of internal and external factors)
• Light is a key exogenous zeitgeber that influences the sleep/wake cycle

  • Light can reset the body's main endogenous pacemakers (SCN), and also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation, etc
  • Campbell and Murphy (1998) woke 15 ppts at various times and shone a light on the backs of their knees - producing a deviation in the sleep/wake cycle of up to three hours
  • Light does not necessarily rely on the eyes to influence the SCN
• Social cues also have an important influence on the sleep/wake cycle

  • The sleep/wake cycle is fairly random in human newborn but most babies are entrained by about 16 weeks
  • Schedules imposed by parents are a key influence, including adult-determined mealtimes and bedtimes
  • Research on jet lag shows adapting to local times for eating and sleeping (not responding to one's own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag
• One limitation is effects of exogenous zeitgebers differ in different environments

  • Exogenous zeitgebers do not have the same effect on people who live in places where there is very little light in winter
  • For instance, the Innuit Indians of the Arctic Circle have similar sleep patterns all-year round, despite spending around six months in almost total darkness
  • This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
• Another limitation is case study evidence undermines effects of exogenous cues

  • Miles et al (1977) reported the case of a man, blind from birth, with an abnormal circadian rhythm of 24.9 hours
  • Despite exposure to social cues, such as mealtimes, his sleep/wake cycle could not be adjusted
  • This suggests that social cues alone are not effective in resetting the biological rhythm and the natural body clock is stronger
• Evaluation extra: Age-related insomnia

  • Natural changes in circadian rhythms mean older people fall asleep early and have poorer quality sleep (Duffy et al 2015)
  • However, Hood et al (2004) found that management insomnia improved if elderly people were more active and had more exposure to natural light during the day
  • This suggests that exogenous changes in lifestyle may be just as likely to cause age-related insomnia as internal, biological changes