Endogenous pacemakers and exogenous zeitgebers

• Endogenous pacemakers and the sleep/wake cycle

  The suprachiasmatic nucleus (SCN)

  Located in hypothalamus. One of the primary endogenous pacemakers and influential in maintaining circadian rhythms such as the sleep/wake cycle. Receives info about light directly from optic chiasm (area where nerve fibres connected to the eye cross on there way to visual area of cerebral cortex). Continues when eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.
• Endogenous pacemakers and the sleep/wake cycle 2

  Animal studies and the SCN

  DeCoursey et al. (2000) destroyed SCN connections in the brains of 30 chipmunks who were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and by the end of the study a significant proportion had been killed by predators (awake and vulnerable to attack when should have been asleep).
• Endogenous pacemakers and the sleep/wake cycle 3

  Animal studies and the SCN 2

  Ralph et al. (1990) bred 'mutant' hamsters w/ 20-hour sleep/wake cycle. When SCN cells transplanted to brains of normal hamsters, cycles defaulted to 20 hours.

  Both studies emphasise role of SCN in establishing and maintaining the circadian sleep/wake cycle.
• Endogenous pacemakers and the sleep/wake cycle 4

  The pineal gland and melatonin

  The SCN passes info on day length and light to the pineal gland. During the night, increases production of melatonin (a chemical that induces sleep and is inhibited during periods wakefulness). Melatonin has also been suggested as a causal factor in SAD.
• Exogenous zeitgebers and the sleep/wake cycle

  Exogenous zeitgebers are external factors in the environment that reset our biological clocks through a process known as entrainment. In absence of external cues, the free-running biological clock that controls the sleep/wake cycle continues in a distinct cyclical pattern. So sleeping and wakefulness would seem to be determined by an interaction of internal and external factors.
• Exogenous zeitgebers and the sleep/wake cycle 2

  Light

  Can reset the SCN (main endogenous pacemaker) and so plays role in maintenance of the sleep/wake cycle.

  Campbell and Murphy (1998) demonstrated that light may be detected by skin receptor sites on the body even when the same info isn't received by the eyes. 15 ppts were woken at various times and a light pad was shone on the back of their knees. Produced a deviation in sleep/wake cycle of up to 3 hours in some cases. Suggests light is a powerful exogenous zeitgeber which may not rely on eyes to exert its influence on the brain.
• Exogenous zeitgebers and the sleep/wake cycle 3

  Social cues

  In human infants, the initial sleep/wake cycle is pretty much random. Circadian rhythms begin at about 6 weeks and most babies are entrained by about 16 weeks. Schedules imposed by parents are likely a key influence, including adult-determined mealtimes and bedtimes.
• Endogenous pacemakers and exogenous zeitgebers- evaluation

  There may be many other complex influences on the sleep/wake cycle, aside from the master clock (SCN). Research has revealed there are numerous circadian rhythms in many organs and cells in the body, called peripheral oscillators. Although influenced by actions of SCN, they act independently. Damiola et al. (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the SCN unaffected.
• Endogenous pacemakers and exogenous zeitgebers- evaluation 2

  Animal studies have ethical issues. In the DeCoursey et al. study the animals were exposed to considerable harm, and subsequent risk, when they were returned to their natural habitat. There is debate whether what we learn justifies the aversive procedures.
• Endogenous pacemakers and exogenous zeitgebers- evaluation 3

  The influence of exogenous zeitgebers may be overstated. Miles et al. (1977) recount the story of a young man, blind from birth, w/ a circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep/wake cycle could not be adjusted, and consequently, he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world. Suggests that there are occasions when exogenous zeitgebers may have little bearing on our internal rhythm.