Biological Rhythms

• Biological rhythms

  • exert an important influence on the way in which body systems behave
  • governed by 2 things: endogenous pacemakers (body's internal biological 'clocks') and exogenous zeitgebers (external changes in the environment)
  • some of these rhythms occur many times a day (ultradian), others take longer than a day (infradian) and some take much longer (circannual)
• Circadian rhythms

  • last around 24hrs e.g sleep/wake cycle - we feel drowsy when it's night-time and alert during the day demonstrates the effect of daylight
  • Siffre's cave studies - spent several extended periods underground to study the effects in his own biological rhythms. Deprived of natural light + sound, but access to adequate food + drink. His 'free-running' biological rhythm settles down to one that was just beyond 24hrs though he did continue to fall asleep and wake up on a regular schedule
• Infradian rhythms

  • longer than a day but less than a year e.g menstrual cycle
  • Mclintock demonstrated how menstrual cycles may synchronise a a result of the influence of female pheromones - 29 women with irregular periods - samples of pheromones were gathered from 9 of them via a cotton pad in their armpit after being worn for 8hrs. The pads were treated with alcohol + frozen, then were rubbed on the upper lip of the other women. She found that 68% of the women experienced changes to their cycle. This suggests that synchronisation has occurred in 2/3 of the sample
• Infradian rhythms

  • seasonal affective disorder (SAD) - depressive disorder which has a seasonal pattern of onset and is described + diagnosed as a mental disorder in the DSM-V
  • main symptoms are low mood and a lack of activity and interest in life
  • psychologists have suggested that melatonin is implicated in the cause of SAD. During the night, the pineal gland secretes melatonin until dawn when there's an increase in sunlight. During winter, the lack of light in the morning means the secretion continues for longer. This is thought to have a knock-on effect on the production of serotonin
• Ultradian rhythms

  • several times a day e.g sleep cycle
  • psychologists have identified 5 distinct stages of sleep that are repeated throughout the course of the night.
  • Stages 1 + 2: 'sleep escalator', light sleep where the person may be easily woken. Brainwave patterns start to become slower and more rhythmic (alpha waves), becoming even slower as sleep becomes deeper (theta waves)
  • Stages 3 + 4: delta waves which are slower. This is deep/slow wave sleep and is difficult to wake
  • Stage 5: REM sleep, body is paralysed but brain activity speeds up, this stage is linked with dreaming
• Endogenous pacemakers and the sleep/wake cycle - Suprachiasmatic nucleus (SCN)


  • tiny bundle of nerve cells in the hypothalamus in each hemisphere which are influential in maintaining circadian rhythms e.g sleep/wake cycle
  • nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebra; cortex, the SCN lies just above the optic chiasm
  • it receives information about light directly from this structure, it continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight when we're asleep
• Human studies of the SCN - Ao3

  • Siffre's cave studies - spent several extended periods underground, deprived of natural light + sound, but access to adequate food + drink. His 'free-running' biological rhythm settles down to one that was just beyond 24hrs though he did continue to fall asleep and wake up on a regular schedule
  • Aschoff + Wever - bunker study - ppts spent 4 weeks in a WWII bunker, deprived of natural light. All but one displayed a circadian rhythm between 24-25hrs. These studies suggest that the SCN still controls our sleep/wake cycle, even in the absence of light
• Animal studies of the SCN

  • DeCoursey destroyed the SCN connections in the brains of 30 chipmunks and then returned to their natural habitat and observed for 80 days. The sleep/wake cycle disappeared and a significant proportion of them had been killed by predators as they were vulnerable to attack when they should have been asleep. This suggests that the SCN has a powerful influence in the sleep/wake cycle in animals. However there is a problem of anthropomorphism.
• Pineal gland and melatonin

  The SCN passes the information on day length and light that it receives to the pineal gland. During the night, the pineal gland increases production of melatonin which induces sleep and is inhibited during periods of wakefulness
• Exogenous zeitgebers and the sleep/wake cycle

  • sleeping and wakefulness is determined by an interaction of internal and external factors
  • Light - can reset the body's main endogenous pacemaker, the SCN, thus plays a role in the maintenance of the sleep/wake cycle. Campbell + Murphy demonstrated that light may be detected by skin receptor sites. 15 ppts woken at different times and a light pad was shone on the back of their knees. They managed to deviate the ppt's sleep/wake cycle. This suggests that light is a powerful exogenous zeitgeber
• Exogenous zeitgebers and the sleep/wake cycle

  • Social cues - initial sleep/wake cycle is random, at about 6 weeks, the circadian rhythms begin and by about 16 weeks, most babies are entrained. The schedules imposed by parents are likely to be a key influence, including adult-determined mealtimes and bedtimes Research also suggests that adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag