circadian rhythm

• biological rhythm 


  cyclical changes in the way the biological systems behave = occurring in cycles

  these rhythms evolved due to cyclical changes in the environment (day & night, winter & summer)
• circadian rhythm 


  most important type of rhythm , a cycle that repeats every 24 hours (sleep/wake or temperature)
• word comes from Latin

  circa = about , diem = day

  nearly all biological organisms have a biological 24 hours a day
• external / internal signals

  external = light & darkness are external signals that determine when to sleep and wake - exogenous zeitgebers

  internal = this rhythm also dips and rises at different times in the day due to our internal body clock - endogenous pacemakers
• endogenous pacemakers : SCN (the suprachiasmatic nucleus)= internal signals


  tiny clusters of nerve cells within the hypothalamus

  acts as the 'master clock' and links to other brain regions that control sleep and arousal

  nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex

  the SCN lies above the optic chiasm and receives information about light directly from it
• endogenous pacemakers 2


  thus continues when our eyes are closed - through eyelids - allowing the biological clock to adjust to changing patterns of daylight whilst we sleep

  if bio clock is running is running slow (sun rises earlier than the previous day) the morning light automatically adjusts our body clock
• endogenous pacemakers - pineal gland 


  the SCN sends signals to the pineal gland

  at night the pineal gland increases production of melatonin

  melatonin levels decrease in the morning when it becomes light
• endogenous pacemakers - practical application 


  insomniacs treated with melatonin find it easier to get to sleep , can be used as a treatment for let lag
•  light detected by eyes -> info about light travels along optic nerve , crosses in optic chiasm on way to visual cortex -> the SCN above the optic chiasm and picks up info about light/darkness -> the SCN sends message about light levels to pineal gland -> if dark pineal gland stimulates production of melatonin. if light pineal gland inhibits the production of melatonin
• exogenous zeitgebers - external signals 


  environmental events that are responsible for resetting the biological clock (light, social cues - meal times)
• exogenous zeitgebers - light 


  light is main exogenous zeitgebers which can reset the interal body clock - because receptors in the SCN are sensitive to changes in light.

  light doesn't just play a role in the sleep/wake cycle - if also influences bodily processes such as hormone secretion and blood circulation
• exogenous zeitgebers - light - study


  study even found that light may be detected by skin receptor sites on the body even when same if isn't received by the eyes

  15 p's were woken at various times and a lighy pad was shone on back of their knees - researchers managed to produce a deviation in sleep/wake cycle by up to 3 hours - shows light is very powerful EZ
• exogenous zeitgebers - social cues 


  • meal times and social activities

  circadian rhythms begin at about age of 6 weeks , by about 16 weeks the rhythms are set

  likely due to schedules imposed by parents

  jet lag can be reduced by adapting to local times for eating and sleeping
• conclusion

  both SCN and pineal gland are endogenous pacemakers which work together to control our sleep/wake cycle

  but both SCN and pineal gland are dependent on the light/dark rhythms of the outside world despite being internal mechanisms = so endogenous pacemakers and exogenous zeitgebers must play a role in sleep/wake cycle
• supporting evidence for SCN


  Morgan removed SCN of hamsters and found their circadian rhythms disappeared - suggesting that the SCN is the endogenous pacemaker

  but it could be the operation itself which caused damage to other areas that controlled the cycle - further evidence needed
• supporting evidence for SCN - further evidence 


  Morgan then transplanted SCN cells from foetal hamsters into the hamsters and rhythms were re-established

  also transplanted SCN cells from mutant hamsters with shorter cycles - 20 instead of 24 hours- and found the transplanted hamsters took on the mutant cycles
• Morgan hamster study criticism 


  used animals - cant generalise the findings to humans
• supporting evidence for pineal gland 


  Schochat et al

  6 male p's spent 29hrs in a sleep lab - between 7am one day and noon following day

  spent 7 mins in every 20 lying down in darkness trying to sleep

  • tendency to sleep (sleep propensitu0 was measured at different times throughout the day
  • levels of melatonin measured by blood samples throughout session

  = key finding , close relationship between tendency to sleep and levels of melatonin - greatest sleep propensity started in late evening - darkness stimulating melatonin
• Schochat sleep study - strength 


  takes place in well controlled environment - sleep lab
• Schochat sleep study - weakness 


  a correlation does not imply cause and effect - cant say melatonin causes sleep propensity as sleep propensity might increase melatonin levels
• strength - evidence fir importance of exogenous zeitgebers - cave case study

  Siffre - spent long periods of time in a cave - no external clues to guide circadian rhythm - he woke, ate and slept when he thought it was appropriate to do so

  his circadian rhythm became 25 hour cycles rather than normal 24.

  also thought it was 20th august but was actually 17th September - suggests that light and other social cues (which he was deprived of) are important in regulating our circadian rhythms not just the internal body clock
• critisism of exogenous zeitgebers 


  the influence might have been overstated - studies of people who live in artic circle (sun doesn't set in summer months) show normal sleep patterns despite prolonged exposure to light. - suggests there are occasions that exogenous zeitgebers may have little bearing on circadian rhythms - could be argued that in these circumstances other external cues take over and influence internal body clock instead of light
• general strength of circadian rhythm 


  has important implications in work place

  (reduced concentration on early hours - strongest drive to sleep is 2-4am)

  shift workers are 3x more likely to develop heart disease - stress to adjusting to different sleep/wake patterns

  • economic implications with reducing illness and how to manage worker productivity
• general criticism of circadian rhythm 


  there are individual differences in circadian rhythms - cycle length can vary from 13-65 hours

  some people show natural preferences for going to bed early (larks) and opposite (owls)

  also age differences in sleep/wake patterns
• general criticism of circadian rhythm


  problem with the research is most studies use small studies and case studies - sample might not be representative so studies may lack population validity

  siffre - returned to cave at 60 - internal body clock ticked slowly than when he was younger - even when same person involved - many factors ca effect circadian rhythms - generalisations cant be made
• conclusion

  both SCN and pineal gland are dependent on the light/dark rhythms of outside world despite being internal mechanisms - both endogenous pacemakers and exogenous zeitgebers must play a role in our sleep wake cycle