Pacemakers and Zeitgebers

• Endogenous Pacemakers

  • Internal ‘biological clocks’ that manage our rhythms (eNdo = iNternal).
  • The main pacemaker in mammals is the Suprachiasmatic Nucleus (SCN).
  • This is a cluster of nerve cells located in the hypothalamus. 
  • This detects sunlight and alters our sleep/wake cycle to put our biological rhythm in time with the environment. 
  • It does this by stimulating the pineal gland to produce more melatonin which induces sleep by inhibiting the mechanisms that create wakefulness.
• Exogenous Zeitgebers

  • Exogenous zeitgebers are external cues that help to keep biological rhythms in tune with the changing external environment. (eXogenous = eXternal)
  • Exogenous zeitgebers are crucial in keeping the biological rhythms in time with that of the changing external environment. 
  • Examples - sunlight, noise, social interaction.
• Disrupted Sleep

  • Our pacemakers always try to impose the inbuilt rhythm of sleep.
  • Our zeitgebers need to be balanced with our pacemakers for a healthy sleep/wake cycle to occur. 
  • Disruption to our biological rhythms has been shown to lead to: disrupted sleep patterns, increased anxiety, decreased alertness, decreased vigilance.
  • Entrainment is the adjustment of an endogenous pacemaker to synchronise with an external zeitgeber.
• De Coursey

  • Removed the Suprachiasmatic Nucleus in 30 chipmunks and compared them to a group of chimpunks with an intact SCN.
  • Returned them to their natural habitat
  • 80 days later: increase in chipmunk death amongst group with no SCN. 
  • The chipmunks were likely to have stayed awake in their burrows at night and therefore be under more danger of predation due to the noise created.
  • It is important to note however that animals have less social cues affecting their sleep/wake cycle so extrapolation to humans needs to be considered with caution. 
• Miles et al - Blind man whose sleep wake cycle was almost 25 hours, despite social cues and noise being readily available. He had to regulate sleep using sedatives. Light may be pretty important for regulating sleep. 

  Campbell and Murphy - Shone a light onto the back of the knee of 15 people at various times throughout the night. Their sleep/wake cycles changed. Light is a powerful exogenous zeitgeber and can be detected via the skin as well as the eye.