EXOGENOUS ZEITGEBERS

• An external, environmental cue e.g. light and social cues that help regulate our biological rhythms through the process of entrainment. 
• Light 


  Receptors in the SCN are sensitive to changes in light levels during the day and use this information to help synchronise activity. Light can reset the body's endogenous pacemaker, the SCN which maintains the sleep/wake cycle. 
• Campbell and Murphy (1998)

  • Wanted to investigate how powerful light is as an exogenous zeitgeber and whether it can be detected any other way besides the eyes. 
  • 15 participants were woken at various times with light being shone at the back of their knees and there were changes in the sleep wake cycle of changes up to 3 hours, proving that light is a powerful exogenous zeitgeber and it does not need to enter our eyes to affect our sleep cycle. 
• Social cues


  Social stimuli such as mealtimes and social activities can act as zeitgebers. 
• Aschoff et al (1971) showed that individuals can compensate for the absence of zeitgebers like natural light by responding to social zeitgebers. 
• Klein and Wegmann (1974) found that circadian rhythms of air travellers can be adjusted more quickly if they went outside more at their destination. This is because they were exposed to the social cues of their new time zone which acted as a zeitgeber. 
• In infants the sleep/wake cycle is initially random but by 6 weeks their circadian rhythm begins and by 16 weeks most babies are entrained. Parents can establish a routine to help entrain their child into a sleeping pattern e.g. mealtimes, making it dark, keeping to a routine. 
• Strength

  ✅Practical application to help avoid jet lag – by understanding exogenous zeitgebers we can entrain our circadian rhythms to fit into changing time zones to reduce the impact of jet lag to people who travel a lot. 
• Limitation


  ❌Case study evidence that challenges the role of exogenous zeitgebers – Miles et al recounted an incident of a man who was born blind with a circadian rhythm of 24.9 hours. Despite exposure to social cues, it could not be adjusted, and he had to take sedatives and stimulants to keep up with the world. This implies social cues alone are not effective in resetting biological rhythms.