infradian rhythms

• • Infradian rhythms - biological rhythms that last longer than 24hrs - e.g- menstrual cycle
• Weekly infradian rhythms -

  • There are some observed differences in human behaviour that conform to a weekly cycle 
  • Eg - male testosterone elevated at the weekends 
  • Young couples report more sexual activity at weekends than weekdays
  • however, this evidence is not the most reliable
• Monthly infradian rhythms - 

  • Female reproductive cycle is known as the menstrual cycle - lasts a month
  • However there are variations in this cycle - some 23 days some 36 days
  • Cycle is regulated by hormones - either to promote ovulation or stimulate the uterus for fertilisation 
• Synchronising the menstrual cycle-

  • Cycle in an endogenous system but has been found to be influenced by exogenous factors
  • Stern and Mclintok - studies 29 women with irregular periods - took samples of pheromones form 9 of the women at different cycle stages 
  • The women had to wear a cotton pad in their armpit for 8 hrs - then frozen and rubbed on upper lip of other participants
  • Day 1 - all 20 women treated with day 1 pheromones - this was repeated 
  • Found that 68% of women experienced changes to their cycle that bought them closer to their 'odour donor' 
• • Infradian rhythms can explain menstrual synchrony by natural selection - evolutionary purpose of getting pregnant at the same time - women can raise babies together - if any babies lose their mothers the other mothers can care for them - adaptive strategy 
  • Problem of confounding variables - stress, diet changes and exercise - all not controlled in menstrual cycle studies - it is possible that any supposed pattern of synchronisation is nothing but chance - there has been failed attempts to replicate Stern's research 
• Annual infradian rhythms - 

  • Seasonal affective disorder - depressive disorder with seasonal pattern 
  • Characterised by low mood and lack of interest in life 
  • Triggered by the shorter number of daylight hours in the winter months 
  • Can also be classed as a circadian rhythm as SAD may be due to disruptions in the sleep/wake cycle and prolonged periods of daily darkness
  • Attributed to be slower melatonin release due to lack of light - knock-on-effect on serotonin production in the brain - linked to depression 
• • Application of Infradian Rhythms - helping to treat SAD - light therapy treatments - helps reset body's internal clock - but this can induce headaches 
  • Sanassi et al - light therapy reduces effects of SAD in 80% of people