Infradian and ultradian rhythms

• examples of infradian rhythms:

  female menstrual cycle and SAD
• the human female menstrual cycle is about 28 days (i.e. less than one cycle in 24 hours)

  rising levels of oestrogen cause the ovary to develop and release an egg (ovulation). then progesterone helps the womb lining the thicken, readying the body for pregnancy. if pregnancy doesn't occur the egg is absorbed into the body and the womb lining comes away and leaves the body (menstrual flow)
• what study showed exogenous zeitgebers may synchronise menstrual cycles?


  stern and mcclintock
• stern and mcclintock's study:

  studied 29 women with irregular periods. pheromones were taken from some at different stages of their cycles, via a cotton mad under their armpits. these pads were cleaned with alcohol and later rubbed on the upper lips of the other participants.

  68% of women experienced changes to their cycle which brought them closer to the cycle of their 'odour donor'
• seasonal affective disorder (SAD) is a depressive disorder (low mood, lack of activity) with a seasonal pattern

  often called the 'winter blues' because the symptoms are often triggered during the winter months when the number of daylight hours becomes shorter

  SAD is an infradian rhythm called a circannual (yearly) cycle
• potential cause of sad:

  during the night, the pineal gland secretes melatonin until daw when there is an increase in light

  during winter, the lack of light in the morning means secretion goes on longer

  this has a knock-on effect on the production of serotonin in the brain (low serotonin is linked to depressive symptoms)
• What is an example of an ultradian rhythm?


  the stages of sleep
• stages of sleep:

  sleep pattern occurs in 90-minute periods (i.e. more than one cycle in 24 hours)

  divided into five stages, each characterised by a different level of brainwave activity (monitored using EEG)
• stages of sleep:

  1&2: light sleep. person may be easily woken. brainwaves become slower and more rhythmic (alpha waves), slowing further as sleep becomes deeper (beta waves)

  3&4: difficult to rouse someone. deep sleep or slow wave sleep characterised by delta waves which are slower still and have a greater amplitude

  5: REM sleep. fast, jerky activity of eyes. body paralysed yet brain activity speeds up
• limitation: methodology used in synchronisation studies

  commentators argue that there are many factors that may change a woman's menstrual cycle and act as confounding variables in research (e.g. stress, diet). so any pattern of synchronisation (e.g. in stern and mcclintock's study) is what we would expect to occur by chance. also, research involves small samples of women and relies of them self-reporting onset of their own cycle (may be inaccurate). this suggests important aspects of synchronisation studies may lack validity.
• strength: evidence supports sleep stages

  a landmark study by dement and kleitman monitored the sleep patterns of nine participants in a sleep lab and found evidence for stages of sleep, specifically REM sleep. REM activity during sleep correlated with dreaming: brain activity varied according to how vivid dreams were; participants woken during dreaming reported accurate recall of their dreams. the study suggests that REM (dreaming) sleep is a distinct ultradian rhythm and an important component of the ultradian sleep cycle
• limitation: use of animal studies

  the role of pheromones in animals sexual behaviour is well-documented and is the basis for most of our knowledge. for example, sea urchins release pheromones into the surrounding water so other urchins eject their sex cells simultaneously. in contrast, evidence for the effects of pheromones on human behaviour remains speculative and inconclusive
• strength: practical application

  an effective treatment for SAD is phototherapy, a light box that simulates strong light in the morning and evening (thought to reset melatonin levels). this relieves symptoms in up to 60% of sufferers. But the same study recorded a placebo effect of 30% using a 'sham negative-ion generator'. this casts doubt on the real value of phototherapy, which may only be effective because of expectations.