Infradian and ultradian rhythms

• Infradian rhythm= type of biological rhythm with a frequency of less than one cycle in 24 hours. Eg: menstrual cycle, seasonal affective disorder.
• Synchronising the menstrual cycle:

  • Evidence suggests that menstrual cycle may be influenced by exogenous factors, like cycle of other women.
  • Stern and McClintock demonstrated how menstrual cycles may synchronise as a result of the influence of pheromones.
  • Samples of pheromones were gathered from 9 women at different stages of their cycle via a cotton pad in their armpit.
  • Pads were rubbed on the upper lip of other participants.
  • Found 68% of women experienced changes to their cycle which brought them closer to their pheromone donor.
• Pheromone= odourless chemical substance that we releases into the environment that effects the behaviour of others 
• Seasonal affective disorder= a depressive disorder which has a seasonal pattern of onset.

  • Symptoms= persistent low moods, lack of activity and interest in life.  
  • Triggered during winter months when daylight hours become shorter.  
  • Type of infradian rhythm called a circannual rhythm (yearly cycle)  
  • Can be classed as a circadian rhythm as experience of SAD may be due to disrupted sleep/wake cycle as this can be attributed to prolonged period of daily darkness.  
• • Hypothesised that the hormone melatonin is implicated in the cause of SAD.
  • During night, pineal gland secretes melatonin until there is an increase in light.
  • During winter, the lack of light in the morning means this process continues for longer.
  • Though to also have an effect on the production of serotonin in the brain- chemical thats been linked to onset of depression.
• Infradian evaluation- methodological limitations

  There are many factors that may effect change to the menstrual cycle, including stress, changes in diet, exercise.

  • There may be confounding variables which means that any supposed pattern of synchrony is no more than would have been expected to occur by chance.
  • This may explain why other studies have failed to replicate the findings.
  • Suggest that menstrual synchrony studies are flawed.
• Ultradian rhythms= type of biological rhythm with a frequency of more than one cycle in 24 hours. Eg: stages of sleep

  • Identified 5 distinct stages of sleep that span around 90 mins- they cycle continues throughout the night.
  • Each stage is characterised by a different level of brainwave activity which can be monitored using an EEG.
• Stages 3 and 4= known as deep sleep or slow wave sleep. The brain waves are deep waves with lower frequency and higher amplitude. Difficult to wake someone in this stage.

  Stages 5 (REM)= body is paralysed yet brain activity closely resembles that of the awake brain. Brain produces theta waves and the eyes occasionally move around. Dreams experienced during REM sleep.
• Stages 1 and 2= this is light sleep where a person may be easily woken.

  Stage 1= brain waves are high frequency, have a short amplitude and are alpha waves.

  Stage2= the alpha waves continue but there are occasional random changes in pattern= sleep spindles.
• Ultradian evaluation- improved understanding

  Strength= improved understanding of age-related changes in sleep.

  • Observed that SWS reduces with age. Growth hormone is mostly produced during SWS therefore this is reduced in older people. According to Eve van Cauter the resulting sleep deficit may explain various issues in old age, such as reduced alertness.
  • In order to increase SWS, relaxation and medication may be used.
  • Suggests that knowledge of ultradian rhythms has practical value.
• Ultradian evaluation- individual differences

  Limitation= significant variation between people.

  • Tucker found large differences between participants in terms of the duration of each sleep stage, particularly stages 3 and 4.
  • Tucker suggested these differences are likely biologically determined
  • So its difficult to describe 'normal sleep' in any meaningful way.
• Infradian evaluation- evolutionary basis

  Strength= menstrual synchrony can be explained by natural selection.

  • For distant ancestors it may have been advantageous for women to menstruate together and become pregnant at the same time.
  • In a social group it would allow babies who have lost their mother to access to milk, therefore improving their chances of survival.
  • So synchronisation is an adaptive strategy.