9. endogenous pacemakers & exogenous zeitgebers

what is the sleep/wake cycle

a daily cycle of biological activity based on a 24 hour period (circadian rhythm) that is influenced by regular variations in the environment, such as the alternation of night and day

what is an endogenous pacemakers

internal body clocks that regulate many of our biological rhythms, such as the influence of the SCN on the sleep/wake cycle

what is the suprachiasmatic nucleus

• tiny bundle of nerve cells in the hypothalamus in each brain hemisphere

• maintains circadian rhythms eg sleep/wake cycle

• nerve fibres connected to the eye cross in the optic chiasm on the way to the left and right area of the cerebral cortex

• receives info about light from the optic chiasm and continues when eyes are shut enabling biological clock to adjust to changing patterns of daylight whilst we are asleep

animal studies on the SCN

decoursey - chipmunks

• destroyed the SCN connections in the brains of 30 chipmunks

• returned to natural habitat and observed for 80 days

findings = the sleep/wake cycle disappeared and by the end a significant proportion had been killed by predators (presumably because they were awake, active and vulnerable to attack when they should have been asleep

the pineal gland

• the SCN passes the info on day length and light that it receives to the pineal gland (behind hypothalamus)

• its another endogenous mechanism guiding the SWS

• during the night it increases production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness

• melatonin has also been suggested as a casual factor in SAD

what are exogenous zeitgebers

external factors that affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle

what can light do (findings)

• can reset the bodys main endogenous pacemaker, the SCN, and thus plays a role in the maintenance of the sleep/wake cycle

• has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation

study on light

campbell and murphy

• demonstrated light may be detected by skin receptor sites on the body even when the same info is not received by the eyes

• 15 participants were woken at various times and a light pad was shone on the back of their knees

• they managed to produce a deviation in their usual sleep/wake cycle of up to 3 hours

• suggests light is a powerful EZ that need not necessarily rely on the eyes to exert its influence on the brain

social cues as an EZ

1. babies

2. jet lag

babies

• babies are seldom on the same sleep/wake cycle as the rest of the family

• newborn’s are pretty much random

• at 6 weeks they begin and by 16 weeks they have been entrained by the schedules imposed by parents including adult determined mealtimes and bedtimes

jetlag

• research on jet lag suggests adapting to local times for eating and sleeping (rather than responding to ones own feelings of hunger and fatigue) is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances

endogenous pacemakers evalutaion

1. beyond the master clock

2. interactionist system

3. ethics

beyond the master clock

• the SCN does not control all circadian rhythms

• damiola et al found that feeding patterns in mice could shift liver cell rhythms without affecting the SCN - there are peripheral oscillators (eg in the lungs, pancreas, skin) that function independently of the SCN

• circadian rhythms are influenced by multiple b

interactionist system

• endogenous pacemakers cannot be studied in complete isolation

• siffre’s cave study lacked total isolation due to artificial light - in real life pacemakers and zeitgebers factors interact making it hard to separate them for research

• reduces the validity of findings from studies trying to isolate biological influences

ethics

• animal research on the SWS raises ethical concerns

• decoursey et al exposed animals to harm and death upon release into their natural environment - while similar brain structures justify using animals, the harm is a significant ethical issue

• raises question do the scientific benefits outweigh the ethical costs?

exogenous zeitgebers evaluation

1. environmental observations

2. case study evidence

3. age related insomnia

environmental observations

• EZs like light do not affect all people equally

• people living in arctic regions eg inuits of greenland experience extreme variations in daylight yet maintain regular sleep patterns - suggests the SWS is more strongly influenced by EPs than by environmental light cues

• strengthens the role of internal mechanisms in controlling circadian rhythms, reducing reliance on light based interventions for sleep disorders

case study evidence

• social cues may not be effective in resetting the biological rhythm

• miles et al studied a blind man with a 24.9 hour circadian rhythm that could not be reset by social cues eg mealtimes - shows that without visual light perception, social/environmental factors cannot fully regulate the biological clock

• suggests the primacy of EPs over EZs in maintaining circadian rhythms especially in cases of sensory deprivation

age related insomnia

• older adults experience poorer sleep due to changes in circadian rhythms

• duffy et al found older people fall asleep earlier and have more disrupted sleep patterns - changes in the circadian system with age may reduce sleep quality. HOWEVER others suggest health and psychological conditions may also be responsible

• raises debate about whether endogenous changes (natural biological aging) or exogenous factors (health, environment) better explain age related sleep issues