Endogenous pacemakers and exogenous zeitgeist’s

• Sleep/wake cycle


  Occurs approximately every 24 hours and is termed the circadian rhythm
• Circadian rhythm


  • Affected by internal (endogenous) factors e.g our internal body clock
  • Affected by external (exogenous) factors e.g light
• Endogenous pacemaker


  The main one for the circadian rhythm is the suprachiasmatic nucleus (SCN), part of the hypothalamus, which acts as an internal body clock to keep the body on an approximate 24 hour sleep-waking cycle
• SCN is sensitive to light


  It regulates the pineal gland, which secretes melatonin - a hormone which encourages sleep
• Less light


  More melatonin is produced
• More light


  Melatonin levels are reduced and waking occurs
• Evidence for the role of the SCN

  • Menaker lesioned this structure in hamsters and their sleep wake cycle was disrupted
  • Appenzeller's case study on people who had suffered damage to the SCN, most often through the removal of a nearby brain tumour - all suffered disturbances to the sleep wake cycle
• Exogenous zeitgeber


  Light is the most important one
• Evidence for the role of light

  • Siffre spent 6 months in a cave with no natural light and no clocks - his sleep wake cycle extended from the normal 24 hour cycle to between 25-30 hours
  • Aschoff and Wever found that in a group of people isolated from daylight, some maintained their regular sleep wake cycles but others had their own very extreme cycles e.g 29 hours awake followed by 21 hours sleep
• Eskimos often live in permanent daylight or permanent night but can still maintain regular daily sleep wake cycles
• Squirrels will still hibernate, even when kept in lab conditions very different from their natural environment
• Humans can adapt their surroundings e.g offices in Norway use artificial blue light in the winter months to improve alertness
• Humans can use certain strategies to reverse the effects of factors like shift work and jet lag on their sleep wake cycle e.g forward rotating shifts, rapidly changing shifts, making themselves stay awake for longer at new destinations and the use of melatonin tablets to encourage them to sleep