biological rhythms

• biological rhythm - something repeated regularly

  • living things are controlled by a variety of rhythms controlled by internal mechanisms and external factors
  • circadian - daily eg. sleep/wake cycle
  • ultradian - more than once in 24 hours eg. stages of sleep
  • infradian - less than once every 24 hours eg. menstrual cycle
• circadian rhythms

  these rhythms last for about 24 hours eg.

  • sleep/wake cycle
  • body temperature cycle
  • release of hormones such as cortisol and melotonin
• other rhythms

  • infradian - a type of biological rhythm with a frequency of less than one cycle in 24 hours eg. pregnancy
  • ultradian - a type of biological rhythm with a frequency of more than one cycle in 24 hours eg. sleep cycles
• the sleep/wake cycle

  there are two possible reasons for the sleep/wake cycle being 24 hours

  1. endogenous pacemakers - it is governed by an internal body clock eg. the scn
  2. exogenous zeitgebers - it is influenced by external factors eg. light, social cues and meal times

  most research suggests our natural sleep/wake rhythm lasts approximately 25 hours, which may be an adaptive advantage that allows us to adapt and stay up later if we need to in order to survive
• endogenous pacemakers

  endogenous pacemakers (eg. the scn in the hypothalamus) - internal biological clocks that determine when we sleep and wake up

  • people have investigated whether it functions alone or with the aid of exogenous zeitgebers (eg. light, social cues and meal times) to see the effect this has on people's sleep/wake cycles
• isolation studies

  studies that remove exogenous zeitgebers to test the way endogenous pacemakers function

  • siffre - isolated himself in a cave for 6 months
  • aschoff + wever - asked participants to spend several weeks in a wwii bunker

  in both cases participants settled down into about a 25 hour sleep/wake cycle, which supports both endogenous pacemakers as participants had a strong internal clock and settled into a regular rhythm, and exogenous zeitgebers as it shows our natural rhythm is different to the one we conform to in daily life which therefore must be controlled by external factors
• key terms

  • entrainment - the process in which exogenous zeitgebers keep the internal body clock in line with the 24 hour clock
  • free running body clock - the body's endogenous sleep/wake cycle without the influence of any external cues
• miscellaneous research

  • folkyard et al. - isolated 12 people for 3 weeks and attempted to override the endogenous pacemakers by tampering with a clock, but no participants' rhythm changed to match the clock
  • however other factors like light may have more of an effect
  • decoursey et al. - removed the scn of 30 chipmunks and found their sleep/wake cycle disappeared completely ad most were eaten - probably because they were sleeping when they shouldn't have
• biological basis

  the internal sleep/wake cycle is controlled by the ~

  • suprachiasmatic nucleus - located in the hypothalamus, regulates the production of melotonin via a connecting nerve pathway, and also connects to the retinas so the amount of light can indirectly affect the release of melotonin
  • pineal gland - the gland that releases melotonin, which induces sleep by inhibiting brain mechanisms that promote wakefulness

  
• exogenous zeitgebers


  external stimuli eg. light and social cues that impact circadian rhythms

  • research suggests these are important for adjusting endogenous pacemakers to be in synchrony with the day/night cycle
  • allow the flexibility needed with fluctuating light patterns and lifestyles
• light

  light is the most influential exogenous zeitgeber over humans' sleep/wake cycle

  • light can reset the scn through the eyes
  • campbell + murphy - shining a light on the back of the knee can shift circadian rhythms by up to 3 hours
  • cells throughout the body contain proteins that are sensitive to light
• evaluation

  + practical application to shift work which goes against circadian rhythms - increased mistakes and accidents at 6am (circadian trough) and 3x more likely to develop heart disease

  + practical application to getting over jet lag

  + practical application to training babies to sleep

  - individual differences - cycle length can vary from 13 to 65 hours, and the timing of cycles varies between morning and night people

  • teenagers' circadian rhythm starts 2 hours later - research may not be generalisable to all groups