Biological Rhythms

• The three type of biological rhythms are circadian, infradian and ultradian
• Circadian rhythms occurs every 24 hours and an example of this is the sleep wake cycle.
• Infradian rhythms occur less than once every 24 hours an example is the menstrual cycle or SAD
• Ultradian rhythms occur more than once every 24 hours, an example of this is the stages of sleep (90 minutes).
• The three main mechanisms used in controlling biological rhythms are Endogenous pacemakers, suprachiasmatic nucleus and exogenous zeitgebers
• Endogenous pacemakers are internal biological clocks that allows organisms to control their internal rhythms. As a result they set a natural free running rhythm in specific biological or psychological processes.
• Suprachiasmatic nucleus is seen as the main endogenous pacemaker in the body, it is a tiny cluster of neurons located in the hypothalamus in the brain implicated in maintaining the body's rhythms. The SCN can receive information about light levels and respond to this external cue to keep the rhythm in synchrony with the environment.
• Exogenous Zeitgebers means 'external time givers' and refers to any external cues that may influence or entrain our biological rhythms. Light is usually seen as the main exogenous zeitgebers but others may include temperature etc
• The main endogenous pacemaker relating to circadian rhythms is the SCN, a tiny cluster of neurons located in the hypothalamus in the brain. The SCN causes the pineal gland to secrete the hormone melatonin which makes us feel sleepy. This may therefore control the sleep-wake cycle.
• It is thought that circadian rhythms such as the sleep-wake cycle are also influence by exogenous zeitgebers which are 'external time givers'
• it is thought that circadian rhythms such as the sleep-wake cycle are also influenced exogenous zeitgebers which are external time givers such as light temperature or noise. The SCN can respond to light - an exogenous zeitgebers. However, without light, the SCN still controls a rise and fall of melatonin suggesting both endogenous and exogenous factors play a role in controlling these rhythms but to different degrees.
• • Evidence to support SCN being key role in circadian rhythms, Morgan who removed the SCN and the cycle disappeared, then gave it back by transplanting SCN cells from foetal hamsters it came back - clear cause and effect
  • Also transplanted mutant strain and changed to 20hours.
• Siffre lived underground for 6 months, with no cues from the sun as to the time of day, sleep-wake cycle settled into a free running rhytm of 25 hours, suggests endogenous pacemakers are important and so is exogenous zeitgebers, but he is just one person not very generalisable
• Shiffre's findings are supported by Aschoff and Wever, found that participants living in WW2 bunkers had a similar free running cycle of 25 hours. This suggests that Shiffres findings can be generalised.
• All of the research on circadian rhythms may ignore the degree of conscious control that we have, Folklard et al asked participants to sleep in a cave and they were shown a clock sped up to 22 hour rhythm, most could not keep up and reverted back to the longer rhythm but one who adapted. This suggest that out circadian rhythms may only be controlled to a certain extent and that our endogenous mechanisms such as the SCN are the driving force.
• A huge strength bodily rhythms research is practical applications, employers can use it to organise shift patterns to increase productivity and well-being to encourage efficient working. Suggests ongoing research has benefits to workers and therefore improve economy.
• many infradian rhythms are clearly controlled endogenously. For example, the human menstrual cycle is governed by the pituitary gland, releasing hormones which control ovulation. They are also influenced by exogenous zeitgebers though such as light. It is thought light plays a key role for SAD. In terms of the menstrual cycle, it is thought that biochemicals called pheromones (released in sweat) can act like hormones and influence people near by.
• Evidence to support infradian rhythms (Russel et al)

  took samples of sweat from donor group and applied it to upperlip of women in second group.

  The groups were kept separate and yet they cycles became synchronised with their individual donor.

  This supports the idea that pheromones act as an exogenous zeitgebers because they could isolate pheromones as the sole factor.

  However, needs to focus on individual differences, cycle varies between and within women from month to month and over a life time.
• A big strength of research into infradian rhythms is practical applications:

  an increased understanding of SAD has enabled researchers to develop light therapy, used at specific times to 'reset' the SCN and improve mood. This suggests that ongoing research into bodily rhythms can benefit real people's lives.
• Stages of sleep for ultradian rhythms

  1. Stage 1

  2. Stage 2

  3. Stage 3

  4. Stage 4

  5. Stage 5 REM
• Stages of sleep


  • Each stage is characterised by a different level of brain activity, monitored by an EEG
• Stage 1 and 2 (light sleep)


  Can be easily woken, brain wave patterns start to become slower and more rhythmic (Alpha waves), become even slower as sleep becomes deeper (theta waves)
• Stages 3 and 4

  Involve delta waves (slower) known as deep sleep or slow wave sleep and it's difficult to awake people from
• Stage 5 REM sleep


  The body is paralysed yet brain activity speeds up in a manner that resembles being awake, associated with dreams
• The fact that all humans display similar trend suggests it's controlled by endogenous pacemaker
• support for endogenously controlled stages of sleep from Dement and Kleitman:

  Found REM acticity occured on average every 92 minutes, also found by waking up participants during REM = more likely to be having vivid dreams.

  Supports timings of sleep cycle.
• • The overall pattern of sleep stages is consistent however there are some individual differences with Newborn babies spending 80% of sleep in REM compared to 20-25% in adults. Suggesting the stages of sleep are not a simple process but adapt to the developmental needs of the individual.
  • Technology and devices based on understanding sleep stages have been developed. These track sleep and help individuals improve sleep. Avoiding waking in stage 4 stops people feeling groggy and disoriented. This technology leads to happier, healthier and more economically productive population.
• Pheromones as chemical messengers are not accepted widely as a process that happens in humans
• There have been some replications of McClintock (1971) (Russel 1990) with positive results
• There have been studies showing no synchronisation, including Trevathan (1993) showing cohabiting lesbian couples did not synchronise
• The optimal conditions for possible synchronization are not known
• As menstrual cycles can vary in length, women can appear to synchronise just due to their variability in their cycles, something not taken into consideration in all of the studies showing positive results
• Evolutionary theories on menstrual synchronisation

  • Advantageous - a way to stop one male dominating a group and having all the children, being unable to impregnate all the women at once due to synchronisation, leading to genetic diversity in small human groups
  • Maladaptive - women wanting to be fertilised by the male with the best genes, and needing to compete with other women for him if they synchronised