Endogenous pacemakers and exogenous zeitgebers AO1

• Biological rhythms

  •Distinct patterns of changes in body activity that conform to cyclical time periods

  •Can be influenced by internal body clocks or external changes to the environment
• Biological rhythms


  Infradian rhythms – last over 24 hours – e.g. menstrual cycle and seasonal affective disorder (SAD)

  Circadian rhythms - last for about 24 hours e.g. sleep/wake cycle and core body temperature

  Ultradian rhythms – Take less than 24 hours to complete e.g. stages of sleep (sleep cycle)
• Endogenous pacemakers

  internal body clocks that regulate many of our biological rhythms. Such as the influence of the suprachiasmatic nucleus (SCN) on the sleep wake cycle
• exogenous zeitgebers


  external cues that may affect our biological rhythms e.g., social cues, light.
• The suprachiasmatic nucleus (SCN)

  •Main endogenous pacemaker in mammals

  •Tiny cluster of nerve cells located in the hypothalamus of each hemisphere of the brain

  •Influential in maintaining circadian rhythms – sleep/wake cycle
• Pathway for SCN


  It gets dark. Nerve fibers connected to the eye cross an area called the optic chiasm. SCN lies just above the optic chiasm and receives information about the dark. SCN sends a signal to the pineal gland to increase melatonin. Melatonin is a hormone that increases sleep.
• Exogenous zeitgeber - light

  •Light not just detected by eyes but also by other body areas e.g. knees

  •Campbell & Murphy (1998)

  •15 participants woken at various times & a light pad shone on the back of their knees

  •Researchers managed to deviate pp’s usual sleep/wake cycle of upto 3 hours

  •Suggesting light is a powerful EZ which doesn’t rely solely on the eyes to exert its influence on the brain
• Exogenous zeitgeber - social cues

  •Research on jet lag suggests that we should adapt to local times for eating & sleeping (rather than responding to own feelings of hunger/fatigue)

  •It is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances