Biological rhythms: Endogenous and Exogenous Pacemakers

• Endogenous pacemakers - internal body clocks that regulate many of our biological rhythms 
• Endogenous pacemakers are biologically determined as its an mechanism within the body
• The suprachiasmatic nucleus (SCN) - a tuny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain = primary endogenous pacemakers in humans = role > maintain circadian rhythms such as the sleep/wake cycle
• SCN lies just above the optic chiasm = recieves info about light directly from this structure > this continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep
• SCN is an internal body clock that help the body to keep on an approximate 24 hour sleep/wake cycle

Animal studies and the SCN

• Patricia DeCoursey et al - destroyed the SCN connections in the brain of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
• Results: sleep/wake cycle = disappeared > significant proportion of them was killed by predators because they were awake and vulnerable to attack when they should have been asleep
• Martin Ralph et al - bred mutant hamsters with a 20 hours sleep/wake cycle > SCN cells from the foetal tissue of mutant hamster were transplanted into the brains of normal hamsters
• Results: the cycles of the second group defaulted to 20 hours 
• Both studies shows the role of the SCN in maintaining the circadian sleep/wake cycle

The pineal gland and melatonin

• SCN passes the info on day length and light that it recieves to the pineal gland = SCN is sensitive to light and regulates the pineal gland which secretes melatonin - a hormone which seems to induce sleep > when there is less day light, more melatonin is produced
• Melatonin = causal factor in SAD

• Influences outside the body that act like a prompt, which may trigger a biological rhythm
• External factors in the environment that reset our biological clocks through a process known as entrainment 
• Absense of external cues = free running biological clock that controls the sleep/wake cycle continues to tick in a distinct cyclincal pattern (Siffre's study)
• Sleep and wakefulness = determined by an interaction of internal and external factors

Light

• Light can reset the body's main endogenous pacemaker - SCN = play a role in the maintenance of the sleep/wake cycle
• Scott Campbell and Patricia Murphy - Light may be detected by skin receptors sites on the body even when the same info is not recieved by the eyes
• 15 p's = woken at various times and a light pad was shone on the back of their knees
• Results: researchers produced a deviation in the p's usual sleep/wake cycle of up to 3 hours in SOME cases
• Suggests that light is a powerful exogenous zeitgeber that don't just only needs to rely on the eyes to exert its influence on the brain

Social Cues

• Infants = random sleep/wake cycles
• 6 weeks of age, the circadian rhythm begin 
• 16 weeks, most babies are entrained 
• Schedules imposed by parents on the infants are likely to be a key influence, including adult-determined mealtimes and bedtimes
• Research suggests = adapting to local times for eating and sleeping rather than your own hunger or fatigue is an effective way of entraining circadian rhythms and beating jet lag

Beyond the master clock

• There are other influences on the sleep/wake cycle rather than just SCN 
• This can include: circadian rhythms in adrenal gland, lungs, liver. pancreas, thymus and skin = these are called peripheral oscillators because CR are in many organs and cells of the body
• Although these preipheral clocks are influenced by the actions of the SCN =  they can act independently
• Damiola et al - changing feeding patterns in mice = alter the CR of cells in the liver by up to 12 hours = leaving the rhythm of the SCN unaffected

Ethics in animal studies

• Generalisations to humans = issue
• DeCoursey et al study - ethical issues  = animals were exposed to harm and risk when they returned to their natural habitat

Influence of exogenous zeitgebers may be overstated

• Artic regions where the sun does not set during the summer months show normal sleep patterns despite the prolonged exposure to light
• Shoes exogenous zeitgebers may have little bearing on our internal rhythm
• Indivuidual differences

Methodological issues in studies

• Just isolating light does not give us an insight into the many other zeitgebers that influence the sleep/wake cycle, and the extent to which these may interact
• Studies still needs to be replicated = reliability issues
• Confounding variables = limited light expose to the p's eyes

Interactionist system

• Pacemakers and zeitgebers interact, it makes more sense to seperate the two for the purpose of research 
• Instead of isolating one influence as in Siffre's study = study is rare therefore lacking validity