Infradian rhythms

• What are infradian rhythms?

  • Biological rhythms that take longer than 24 hours to complete such as the menstrual cycle and seasonal affective depression
• What is the menstrual cycle?

  • Monthly changes in hormone levels which regulate ovulation, referring to the time between the first day of a woman's period to the day before her next (around 28 days)
  • Rising levels of oestrogen causes ovulation, which is when the ovary develops an egg and releases it
  • After ovulation, progesterone helps thicken the womb lining in preparation for pregnancy
  • If pregnancy does not occur, the egg is absorbed back into the body and the womb lining sheds, leaving the body as blood
  • Mainly endogenous but can be affected by exogenous zeitgebers
• What research was conducted into the effect of exogenous zeitgebers on the menstrual cycle?

  • McClintock & Stern (1998) studied 29 women with irregular periods and collected samples of 9 of their pheromones at different stages of their menstrual cycles via cotton pads in their armpits, worn for at least 8 hours to ensure they were picked up
  • Cotton pads, frozen and treated with alcohol were worn on the upper lips of the other ppts.
  • Found that 68% of women experienced changes to their cycles bringing them closer to the cycle of their 'odour donor'
• What is seasonal affective disorder?

  • Depressive disorder patterned by a seasonal onset, symptoms including persistent low mood and lack of activity/general interest
  • Symptoms are triggered during shorter hours of daylight in the winter months
  • Called a circannual rhythm meaning it is subject to a yearly cycle, but can be circadian as it could be a result of disruption to the sleep/wake cycle due to winter hours
  • Melatonin is secreted by the pineal gland during night until dawn - lack of light in the mornings increases melatonin secretion for a longer time which has a knock on effect for serotonin production, which is lower in people with depression
• What is one strength of research into infradian rhythms?

  • Evolutionary basis: menstrual synchrony can be explained by natural selection
  • May have been advantageous for past women to menstruate and fall pregnant together at the same time, as collective care for infants could improve rates of survival as it would allow babies who had lost their mothers during/after childbirth to have access to breast milk
  • Suggests synchronisation is an evolutionarily adaptive strategy
• What is another strength of research into infradian rhythms?

  • Real-world application: light therapy is one of the most effective treatments for SAD - a box that simulates very strong light resets the body's internal clock
  • Sanassi (2014) found that light therapy has helped reduced the effects of SAD in about 80% of people and is also preferred over antidepressants as it is regarded as more safe
  • However... Rohen et al. (2009) recorded a relapse rate of 46% over successive winters compared to 27% of a CBT group, suggesting it may not be a long-term solution
• What is one limitation of research into infradian rhythms?

  • Poor methodology: there are many other factors that can affect the menstrual cycle such as stress, diet changes, exercise, etc. which act as confounding variables
  • This means any supposed pattern of synchronisation is no more than would have been expected to occur by chance, explaining why other studies like Trevathan et al. (1993) have failed to replicate the findings
  • Suggests menstrual synchrony studies are flawed so we must question the validity of their findings