Biological Rhythms

• A biological rhythm is a cyclical variation over some period of time in physiological or psychological processes.

  • Circadian: a rhythm that occurs once in 24 hours (the sleep/wake cycle)
  • Infradian: a rhythm that occurs less than once every 24 hours / have a duration of over 24hrs – so there is a longer gap between cycles (the menstrual cycle - 28 days)
  • Ultradian: a rhythm that occurs more than once every 24 hours / have a duration of under 24hrs– so there is a shorter gap between cycles (the stages in human sleep - every 90 - 120 mins)
• Circadian rhythms make people’s desire for sleep strongest between midnight and dawn. Sleep/wake cycles are influenced by internal factors such as the suprachiasmatic nucleus, the pineal gland and melatonin and external factors such as light and social cues. This means exposure to light at the wrong times can cause disruption to our circadian rhythms which can cause disrupted sleep patterns, increased anxiety and decreased alertness and vigilance. 
• Siffre's cave study

  • Lived in a cave for extended periods of time: 2 months 1962 / 6 months 1972.
  • Both instances despite the lack of natural light Siffre’s sleep/wake cycle only slightly shifted to once every 25 hours. 
  • This suggests we have a free running biological rhythm that is regulated by internal factors. 
• Siffre Evaluation

  • Siffre’s cave study is of one individual. It is very hard to generalise our understanding of circadian rhythms to the wider population on the basis of one single person’s sleep/wake behaviour. 
  • Additionally, he turned on an artificial light when he was awake, so this could have been influencing his circadian rhythms. It therefore can be argued he was not measuring our free running sleep/wake cycle as it was still being influenced by light. 
• One example of a infradian rhythm is the menstrual cycle. This cycle usually lasts about 28 days. The menstrual cycle is influenced by internal factors such as the pituitary gland, oestrogen and progesterone and external factors such as food, stressors and other women whom you spend time with.
• Stern and McClintock

  • 9 women who had irregular periods wore sweat pads under their armpits at different stages of their periods to collect the pheromones. The pads were treated with alcohol and frozen
  • The pads were then placed on the lips of 20 more women who had irregular periods.
  • Most of the 20 women experienced changes in their cycle that brought them closer to the cycle of the woman whose sweat pad they had been exposed to. 
• Stern and McClintlock

  • There are many factors that influence the human menstrual cycle including stress, diet and exercise. 
  • These extraneous variables, along with the small sample, mean any synchronisation seen within the women may have been due to chance. 
  • This means this research may lack validity and our knowledge of the factors that influence infradian rhythms is limited. 
• One example of an ultradian rhythm is the alternation between REM and NREM sleep during the night. The first cycle usually lasts about 90 mins and then each time it is repeated it gets longer (up to about 120 mins). Usually we experience 5 sleep cycles per night. The stages of sleep are influenced by internal factors such as the brainstem and neuronal activity and external factors such as frequently changing sleep schedules and stress. 
• Dement and Kleitman

  • Studied 9 pps in a sleep lab
  • Recorded their brain activity using an EEG
  • Controlled for the use of caffeine and alcohol
  • Found that pps went through 5 stages of sleep and REM sleep was very important for dreaming 
  • Pps woken during this stage could recall a lot of details about their dream and the amount of brain activity correlated to how vivid their dreams were
• Dement and Klietman Evaluation

  • The sample size in this study is incredibly small. This may mean results may not be able to generalise findings.
  • Nevertheless the research has real world application as understanding more about what occurs within each stage of sleep allows us to develop treatment for sleep disorders such as night terrors. 
  • Kleitman also suggested we experience a similar cycle of rest and activity during the day, which explains why we often need breaks after about 90-120 minutes of activity.