EPs & EZs

• Endogenous Pacemakers are internal body clocks that regulate many biological rhythms
• The Suprachiasmatic nucleus is a tiny bundle of nerves located in the hypothalamus of each hemisphere which maintains circadian rhythms
• Nerve fibres connected to the eye cross at the optic chiasm which lies below the suprachiasmatic nucleus
• Animal studies support the function of the SCN
• Patricia DeCoursey et.al. studied 30 chipmunks who had their SCN connections destroyed
• Patricia DeCoursey et.al found that after 80 days the chipmunks' sleep-wake cycles had disappeared
• After the 80 days a significant proportion of chipmunks had been killed by predators due to the disruption of their sleep cycles
• Martin Ralph et.al bred mutant hamsters with a 20-hour sleep-wake cycle
• Martin Ralph et.al transplanted SCN cells from the foetal tissue of mutant hamster into the brains of normal hamsters
• Martin Ralph et.al. found that the normal hamsters defaulted to a 20-hour sleep-wake cycle
• The Pineal Gland is a pea-like structure in the brain located behind the hypothalamus
• The SCN passes on information about day length and light to the pineal gland which produces melatonin
• Melatonin is a hormone which induces sleep
• Exogenous Zeitgebers are external factors that entrain biological rhythms
• Light acts as an exogenous zeitgeber as it has an indirect influence on key processes e.g. hormone secretion & blood circulation
• Campbell & Murphy found that light may be detected by skin receptors on the back of the knee
• Campbell & Murphy studied 15 participants who they woke at various points in the night to shine a light pad on the backs of their knees
• Campbell & Murphy's experiment showed deviations in sleep-wake cycles of up to three hours
• Social Cues act as exogenous zeitgebers by imposing a schedule
• Newborn babies show social cues as they initially have a random sleep-wake cycle but by 16 weeks become entrained by the parents' imposed schedules
• Research into Jet Lag shows social cues as it suggests that adapting to local eating/sleeping times helps to entrain circadian rhythms
• Numerous other circadian rhythms (peripheral oscillators) can also act independently from the SCN, limiting the theory of EPs
• The Interactionist system limits the theory of EPs as components are hard to totally isolate in experiments
• People living in different environments e.g. inuits of the Arctic circle limit the theory of EZs as they still show regular sleep patterns all year round
• Research into blind people e.g. Laughtom Miles et.al limits the theory of EZs as they found that a young man blind from birth had an abnormal sleep schedule (24.9 hours) despite imposed social cues