biological rhythms

• what are the different types of biological rhythms?

  • circadian
  • infradian
  • ultradian
• Example of circadian rhythm?

  • sleep wake cycle
  • lasts for 24 hours
• Examples of Infradian rhythm

  • menstrual cycle
  • lasts more than 24 hours
• examples of ultradian rhythm

  • stages of sleep
  • lasts less than 24 hours
• What is an endogenous pacemaker?

  • things within the body that regulate biological rhythms
• What is an exogenous zeitgeber?

  • cues in the external environment that inform endogenous pacemakers and regulate biological rhythms
• All biological rhythms are governed by endogenous pacemakers and exogenous zeitgebers
• Which biological rhythm occurs many times during the day?

  • ultradian
• Which biological rhythm takes longer than a day to complete?

  • infradian rhythm
• what is an exogenous zeitgeber that affects our sleep cycle?

  • the amount of day light
• Outline the role of the superchiasmatic nucleus (SCN)

  • an endogenous pacemaker
  • lies above the optic chiasm which provides information from the eye about light
  • influenced by the exogenous zeitgeber of light
• Endogenous pacemakers that control circadian rhythm

  • hormones such as melatonin
  • body temperature
  • metabolism and digestion
  • suprachiasmatic nucleus
• How is melatonin influenced in the sleep wake cycle?

  • darkness can trigger the release of melatonin making you feel more sleepy
• Outline Siffre's research


  • studied the affects of living in a cave without the exogenous zeitgeber of natural light
  • spend 2 months in a came without a clock
  • he found that he maintained a regular sleep wake cycle and circadian rhythm of around 25 hours
• Outline Folkard et al's experiment

  • participants were kept in a gave without natural light source
  • they were told to sleep at 11:45 and wake at 7:45
  • researchers increases the clock speed so the day was only 22 hours
  • despite the faster clocks, all but one participant maintained a consistent 24 hour circadian rhythm
• external zeitgebers that can influence endogenous pacemakers

  • social cues (jet lag)
  • light
• Outline Aschoff and Wever's research

  • participants spent four weeks in a world war two bunker deprived of natural light
  • all but one (extended to 29 hours) displayed a sleep wake cycle of 24 hours to 25 hours
  • suggests that the natural sleep wake cycle is entrained by exogenous zeitgebers
• Outline the menstrual cycle as an example of infradian rhythm

  • the menstrual cycle is governed by monthly changes in hormone level that regulate ovulation
  • rising levels of oestrogen cause the ovary to develop an egg and release it
  • after ovulation progesterone helps the womb lining grow thicker
  • if pregnancy does not occur the egg is absorbed into the body and the womb lining sheds
• How long does the typical menstrual cycle last?

  28 days
• What exogenous factor could influence the menstrual cycle?


  • the cycles of other women
• Outline Stern and McClintock's research

  • studied some women with a history of irregular periods: samples of pheromones were gathered from 9 women at different stages of their menstrual cycle, via a cotton pad placed in their armpit
  • pads were treated with alcohol and frozen and placed on the upper lip of the other women
  • 68% of women experienced changes to their cycle which bought them closer to the cycle of their odour donor
• What are the main symptoms of SAD?

  • persistent low mood
  • lack of activity and interest in life
• Seasonal affective disorder is an example of infradian rhythm as it is subject to a yearly cycle
• • seasonal affective disorder may be caused by melatonin
  • during the night the pineal gland secretes melatonin until dawn
  • during the winter, the lack of light in the morning means this secretion continues for longer
• Why may seasonal affective disorder be characterised by depression-like symptoms?

  • melatonin is released for longer periods of time
  • this is thought to effect the production of serotonin in the brain
• the sleep wake cycle is an example of ultradian rhythm
• Outline the sleep wake cycle

  • there are five distinct stages which last approximately 90 minutes
  • the cycle continues throughout the course of the night
  • each stage is characterised by a different level of brainwave activity which can be monitored using an EEG
• How can brainwave activity be monitored?

  • through EEG
• What is stage 1 and 2 of the sleep wake cycle?

  • light sleep
  • a person might be easily woken
  • brain waves are high frequency and have short amplitude
  • both stages have alpha waves but stage 2 there are random changes in pattern (sleep spindles)
• What is stage 3 and 4 of the sleep wake cycle?

  • deep sleep
  • brain waves are delta
  • lower frequency and higher amplitude
  • it is difficult to wake someone up at this point
• What is stage 5 of the sleep wake cycle?

  • the brain produces theta waves
  • the eyes occasionally move around(rapid eye movement)
  • dreams are most likely to occur