Circadian Rhythms

• Endogenous pacemakers


  internal body clocks that keep body to time
• Exogenous Zeitgebers


  external cues that entrain our internal body clocks
• How to Zeitgebers entrain our EP? 


  altering body clocks to match the enviornment
• Siffre 1962 et al.


  
  

  • 6 months in cave without EZs (natural light, clocks, social cues)
  • EP maintained regular cycle of 25 hours without EZ, cycle longer than usual 24 hours
  • EP for sleep-wake cycle is free running but needs entrainment to maintain 24 hour day-night cycle
• Aschoff & Wever 1976 et al.


  
  

  • 4 weeks deprived of light in bunker
  • one P with 29 hour cycle, majority Ps with 24-25 hour cycle
  • natural sleep-wake cycle entrained by EZs associated with our 24 hour day ( eg mealtimes) therefore causing it to be longer than 24 hours
• Free-Running

  predictable cycle that hasn't been entrained by EZ
• SCN

  suprachiasmatic nucleus, an pacemaker
• where is SCN

  in hypothalamus at the optic chiasm
• function of SCN

  
  

  1. controls pineal gland with electrical impulses
  2. stops producing melatonin by sending light to the pineal gland when detected light
• sleep-wake cycle

  EP lasts over 24 hours without EZ
• Ralph 1990 et al.


  
  

  • swapped SCN between 'mutated' & 'normal' hamsters
  • both groups lost normal sleep-wake cycles - therefore SCN necessary to control cycle
  • mutated hamsters (20h) developed circadian rhythm of normal (24h) hamsters, and vice versa
• AO3 strength - practical application to shift work


  
  

  • more reported accidents at 6am & jet lag desynchronises circadian rhythms causing bodily effects
  • circadian rhythms desynchronised prove adverse effects therefore external factors can disrupt rhythms but body unable to adjust hence cognitive & psychological side effects
• AO3 strengths - practical application to drug treatments


  
  

  • Baraldo 2008 = circadian rhythms lead to guidelines in drug dosage timings
  • can take drug at any time and still be effective at peak time rather than waking up at irregular time to take dose therefore understanding co-ordination of body's basic processes avoid disruption to sleep-wake cycle
• AO3 limitation - individual differences


  
  

  • Duffy 2001 = controlled study of larks (morning) Ps wake 6am - sleep 10pm vs owls (evening) Ps wake 10am - sleep 1am; Czeisler 1999 = cycle vary 13h to 65h
  • people have different circadian rhythms therefore cycles doesn't always have to conform to 24 hours
• AO3 limitation - poor control

  
  

  • Siffre 1962 = used artificial light
  • artificial light as confounding variable entrained his EP - he assumed it had no effect on free running circadian rhythm therefore low internal validity
• animal studies & SCN

  
  

  • Ralph 1990
  • DeCoursey 2000
• DeCoursey 2000 et al.


  
  

  • influence of SCN - 30 chipmunks' SCN connections destroyed and released to natural habitat
  • sleep-wake cycle disappeared and killed by predators as they were awake at night therefore vulnerable when should've been asleep
• how the SCN works

  
  

  • retina's sensory receptors detect light and communicate with sensory neurons
  • neurons send info to SCN and pineal gland is told to either increase or decrease release of melatonin
• pineal gland & melatonin

  
  

  • melatonin only in dark - increased production at night induces sleep
  • SCN prevents melatonin production from pineal gland when light is present therefore body is kept awake
• light as EZ

  
  

  • resets EP and sleep-wake cycle
  • Campbell & Murphy 1998 = 15 Ps awoke with light shone at back of knees therefore light can be detected by skin receptors too
• social cues

  
  

  • 6 week old babies have random sleep-wake cycles; 16 week old babies rhythms are entrained by bedtimes and mealtimes
  • jet lag - overcome by entraining circadian rhythms from long distance travel via adapting to local times for eating and sleeping
• AO3 limitation endogenous pacemakers - environmental observations

  
  

  • Inuit people in Arctic Circle similar sleep patterns all-year round despite 6 months total darkness
  • this suggests that sleep-wake cycle primarily controlled by pacemakers that can override environmental changes in light therefore zeitgebers not have same effect in all environments
• AO3 zeitgebers limitation - contrary evidence case study

  
  

  • Miles 1977 = blind man with abnormal circadian rhythm of 24.9h
  • sleep-wake cycle exposed to social cues (eg mealtimes) yet still unadjusted cycle therefore social cues alone are ineffective to reset biological rhythm
• AO3 limitation pacemakers - cannot be studied alone

  
  

  • Siffre = used artificial light
  • EZs and EPs interact in real life - unnecessary to separate therefore more isolation of EP reduces external validity
• AO3 zeitgebers limitation - ethical issues

  
  

  • DeCoursey 2000 = chipmunks killed
  • being awake instead of asleep made it easier for predators; CA - sleep-wake cycle mechanisms similar across mammalian species, REB - then humans should've been test subject yet we know nobody would consent to participating therefore study violates ethics of protection from harm and should've found alternative