Endogenous pacemakers and exogenous zeitgebers
SCN – endogenous pacemaker (internal body clocks)
- Suprachiasmatic nucleus is a tiny bundle of cells in hypothalamus which helps maintain
circadian rhythms (sleep wake cycle).
- Nerve fibres connected to eye cross at optic chiasm on the way to the visual areas of the
cerebral cortex. SCN lies just above the optic chiasm and receives info about light even
when eyes are closed.
Influence SCN on sleep wake cycle is demonstrated in animal studies:
Decoursey et al (2000)
- Destroyed SCN connection in the brains of 30 chipmunks which were returned to their
natural habitat and observed for 80 days.
- Their sleep/wake cycle disappeared, and many were killed by predators, because they
were awake and vulnerable when they should be asleep.
Ralph et al (1990)
- Bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle.
- SCN cells were transplanted from the foetal tissue of these hamsters into normal
hamster’s brains, which then developed 20-hour sleep cycles.
The penial gland and melatonin
- SCN passes info on day length and light to the pineal gland.
- Pineal gland increases production of melatonin at night. Melatonin induces sleep and is
inhibited during periods of wakefulness.
- Melatonin is suggested as a causal factor in SAD
Exogenous zeitgebers
- External factors that affect our biological rhythms such as the influence of light on sleep
wake cycle.
- Light can reset the SCN and can have an influence on key processes in the body.
Light
- Campbell and Murphy (1998) Woke 15 ppts at various times and shone a light on the
back of their knees.
- Producing deviation in the sleep wake cycle of up to 3 hours
- Suggests skin receptor cells also detect light, doesn’t rely on the eyes to exert influence
over the brain
- Suggests light is a powerful exogenous zeitgeber
Social cues
- Infants develop circadian rhythms at 6 weeks, most babies are entrained by 16 weeks.
- Schedules imposed by parents are a key influence, including mealtimes and bedtimes.
SCN – endogenous pacemaker (internal body clocks)
- Suprachiasmatic nucleus is a tiny bundle of cells in hypothalamus which helps maintain
circadian rhythms (sleep wake cycle).
- Nerve fibres connected to eye cross at optic chiasm on the way to the visual areas of the
cerebral cortex. SCN lies just above the optic chiasm and receives info about light even
when eyes are closed.
Influence SCN on sleep wake cycle is demonstrated in animal studies:
Decoursey et al (2000)
- Destroyed SCN connection in the brains of 30 chipmunks which were returned to their
natural habitat and observed for 80 days.
- Their sleep/wake cycle disappeared, and many were killed by predators, because they
were awake and vulnerable when they should be asleep.
Ralph et al (1990)
- Bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle.
- SCN cells were transplanted from the foetal tissue of these hamsters into normal
hamster’s brains, which then developed 20-hour sleep cycles.
The penial gland and melatonin
- SCN passes info on day length and light to the pineal gland.
- Pineal gland increases production of melatonin at night. Melatonin induces sleep and is
inhibited during periods of wakefulness.
- Melatonin is suggested as a causal factor in SAD
Exogenous zeitgebers
- External factors that affect our biological rhythms such as the influence of light on sleep
wake cycle.
- Light can reset the SCN and can have an influence on key processes in the body.
Light
- Campbell and Murphy (1998) Woke 15 ppts at various times and shone a light on the
back of their knees.
- Producing deviation in the sleep wake cycle of up to 3 hours
- Suggests skin receptor cells also detect light, doesn’t rely on the eyes to exert influence
over the brain
- Suggests light is a powerful exogenous zeitgeber
Social cues
- Infants develop circadian rhythms at 6 weeks, most babies are entrained by 16 weeks.
- Schedules imposed by parents are a key influence, including mealtimes and bedtimes.
