biological rhythms

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circadian rhythms

Human sleep/wake cycle

  • we sleep for 8 hours every 24 hours.
  • controlled by endogenous factors (SCN) and exogenous zeitgebers (light)
  • 2 months in a cave with artifical light and developed a rhythms of 25 hours - shows there are endogenous pacemakers that effect the rhythm

Human body temperature

  •  maintained by homeostasis, peaks in the afternoon and troughs in the early morning
  • determined by the heat produced and lost in the body
  • influenced by external temperature and the SCN  
  • body temperature was monitored by volunteers in a lab experiment who were woken at different times. a light was shone on their knee which effected the rhythm by up to 3 hours depending on when it was shone. the blood or skin may effect this rhythm

Evaluation- the SCN was removed from hamsters and found that all circadian rhythms disappeared. the rhythms were restablished by transplating SCN cells. mutent cells were also transpanted and gave the hamsters short rhythms

- a blind man had a 24.5 hour cycle that became out of sync, simulants and sedatives were needed, social cues didn't work. light acts as time cue to reset the rhythm  

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infradian rhythms

human menstral cycle

  • controlled by hormones released from the pituitary gland and the ovaries
  • FSH and LH are released first which stimulate follicle growth and then oestrogen is released which causes ovulation, progesterone is then released the allow implantation
  • underarm sweat was applied to the lips of women which cause their menstral cycle to syncronise, suggesting pheromones are exogenous zeitgebers


  • exogenous factors are thought to influence the cycle (temperature and food availablility)
  • a squirrel was placed in an artifical environment with light for 12 hours and a temperature of 0'c, the animal still hibernated october- april. its body temp dropped in the 300 day cycle. there must be an endogenous factor influencing the squirrel

Evaluation - some evidence has found no correlation between menstrual cycles in a womens basketball team

- a lot of evidence suggesting there's individual differences in peoples infradian rhythms  

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ultradian rhythms

human sleep cycle

  • change from NREM to REM, a whole cycle lasts 90 minutes
  • EEG studies have found that the brain has different activity for each stage
  • REM - muscles are paralysed but brain activity is similar to when awake
  • NREM - brain waves are in patterns
  • participants woken during REM reported dreaming 80% of the time, those woken during NREM reported 20% of the time
  • biological clock in the pons part of the brain that controlls the cycle, also the same factors that influence the sleep/wake cycle

BRAC (basic rest- activity cycle)

  • 90 minutes per cycle
  • first half the brain has fast brain waves the individual feels alert, the last 20 mins to brain waves slow down and the individual feels daydreamy
  • influenced by lifestyle

Evaluation- a study has found that participants report dreams 70% of the time in NREM - down to dream like thoughts being categorised - observation found a clear eating and drinking patern every 90 minutes for 6 hours

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endogenous pacemakers

suprachiasmatic nucleaus (SCN)

  • located in the hypothalamus abover the optic nerve
  • receives information from light penetrated by the eyelids, and syncs the rhythm with outside 

pineal gland and melatonin

  • when light levels reduce the SCN sends signals to the pineal gland which releases melatonin hormone which induces sleep by inhibiting brain mechanisms

body clock gene

  • PERIOD3 is one clock gene that influence rhythm characteristics, each form of gene has a different circadian pattern, determines whether someones a 'morning person' or not


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exogenous zeitgebers


  • a shift in time zones and light occurs then the rhythms realign to the new cues - entrainment

social cues

  • those who live in the arctic circle cannot use light as a exogenous factor so need other cues light social and work habits
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disruption of biological rhythms

jet lag

  • consequences are tireness, irritability, disorientation and problems concentrating
  • happens when travelling east and west
  • due to a mismatch of pacemakers set for one time zone and zeitgebers set for a different zone
  • it's worse travelling west to east, easier to adjust clocks that are ahead of time because of the natural tendency to lengthen our circadian rhythms

Evaluation - 8 participants travelled between USA and Germany and found adjustment easier for those travelling westbound because of the lengthening of their rhythms

- a lot of individual difference, generalisations cannot be made

- disruption to sleep and hormone patterns led to a cabin crew's cognitive functioning being decreased and mental health problems

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disruption of biological rhythms

shift work

  • fluctuating shift work - employees continually change shifts causing disruption to eating and sleeping, workers are always desynchronised, which will impair concentration and physical performance and increase stress
  • non- fluctuating shift work - employees work consistant night shifts, to workers experience a decrease alertness during a shift around midnight (low cortisol levels) and 4 am (low body temperature). the workers may also experience sleep deprivation because they sleep during the day - up to 2 hours less sleep than nights sleep

Evaluation - nuclear power stations disasters, lorry crashes and oil tanker spills have happened during the night and this could be down to the desynchronising effects of shift work 

- 60% divorce rate for night shift workers, could be due to lack of sleep

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