Exogenous zeitgebers + endogenous pacemakers

• Endogenous pacemakers are internal body clocks that regulate our biological rhythms.
• The SCN is a tiny cluster of nerve cells located in the hypothalamus in each hemisphere of the brain. In mammals, it is the main endogenous pacemaker and acts as the master biological clock which links brain regions that control sleep and arousal. It has control over these clocks to keep the body on approx. 24 hour sleep/wake cycle.
• The suprachiasmatic nuclei need resetting when light levels change. It receives information about light levels via the optic nerve, even when our eyes are closed. This enables the biological clock to adjust to changing patterns of daylight, e.g. in winter v.s. summer
• The SCN sends information on day light and length to the pineal gland that is located just behind the hypothalamus. During the night, the pineal gland increases production of melatonin and decreases it as light levels increase in the morning. Melatonin induces sleep by inhibiting brain mechanisms that promote wakefulness.
• A03- Supporting evidence

  DeCoursey et al

  • destroyed the SCN connections of 30 chipmunks
  • were returned to their natural habitat and observed for 80 days and it was found that the sleep/ wake cycle disappeared and by the end of the study, a significant proportion had been killed by predators
• A03

  Ralpht et al bred mutant hamsters that had a 20 hour sleep/wake cycle. They then transferred the foetal tissue of mutant hamsters into the brain of normal hamsters.

  Found that the cycles of normal hamsters defaulted to 20 hours. This supports the role of the SCN in establishing and maintaining a sleep/wake cycle.
• A03

  There are problems with generalising findings frmo animal studies to humans as the cognitive factors cannot be regarded in the same way. They were also tested in artificial conditions and so they might behave differently in their natural habitat.
• A03

  Potential ethical issues as the animals were exposed to considerable harm and subsequent risk whenr eturned to their natural habitat. A cost v.s. benefit analysis can be drawn from here.
• A03

  Siffre & Aschoff+Wever

  (see infradian rhythms)
• Exogenous zeitgebers are external cues in the environment that may trigger or entertain our biological rhythms.
• Light is the most important exogenous zeitgeber for most animals. It can reset the SCN and helps to maintain the sleep/wake cycle. Rods/cones in the retina of the eye detect light to form visual images, BUT a small number of retina cells contain a protein called melanopsin, which is sensitive to natural light and helps gauge the overall brightness and carries signals to the SCN to reset the internal biological clock.
• Social stimuli can act as exogenous zeitgebers. HUman infants have a random sleep/wake cycle when initially born but then get entrained due to mealtime and bedtimes imposed by parents.
• A03

  Campbell and Murray

  • supported the role of light as an exogenous zeitgeber in the sleep/wake cycle
  • 15Ps woken at various times and a light pad was shone on the back of their knees
  • researchers managed to adjust Ps sleep cycles by up to 3 hours
  • this suggests that light is a powerful exogenous zeitgeber that does not need to rely on eyes to influence the brain
• A03

  Research into exogenous zeitgebers has allowed for insights on how to deal with jet lag. This is caused by a misalignment between circadian rhythms and local destination time. It consists of fatigue, insomnia and dizziness.

  Burgess et al

  • exposure to bright light prior to an east to west flight decreased the time needed to readjust their cycle on local time arrival
  • volunteers participated in one of 3 treatments that helped shift their cycle back by one hour a day over 3 days
  • Ps exposed to continous bright light shifted their circadian rhythm by 2.1 hours.
  • Ps exposed to intermittent bright light shifted their rhythm by 1.5 hours
  • Ps exposed to dim light shifted by 0.6 hours
  • Shows that jet lag could potentially be eliminated
• A03

  Disrupting biological rhythms can have negative consequences, when endogenous pacemakers become out of line with exogenous zeitgebers, the sleep/wake can be disrupted heavily, which can have a negative impact on ability to function.
• A03

  The role of exogenous zeitgebers may be overstated. Studies of people living in Arctic regions where the Sun does not set during the summer months showed normal sleep patterns despite the prelonged exposure to light. Suggests that exogenous zeitgebers have little bearing on internal rhythm.