infradian rhythms (biological rhythms)

• Infradian rhythms:

  • Biological cycles lasting more than 24 hours (less often than once every 24 hours)
• female menstrual cycle = infradian rhythm

  • Human female menstrual cycle is about 28 days (24-35 days)
  • Rising levels of oestrogen cause the ovary to release an egg (ovulation)
  • Then progesterone helps the womb lining to thicken, readying the womb for pregnancy
  • If pregnancy does not occur = the egg is absorbed and the womb lining comes away (menstrual flow)
• EZs may synchronise menstrual cycles

  • Stern and McClintock = studied 29 women with irregular periods
  • Pheromones were taken from some at different stages of their cycles (via a cotton pad 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 in their cycle which brought them closer to the cycle of their ‘odour donor’
• seasonal affective disorder (SAD) = infradian rhythm

  • SAD is a depressive order (low mood, lack of activity) with a seasonal pattern
  • symptoms are triggers during the winter months when the number of daylight hours becomes shorter (‘winter blues’)
  • SAD may be caused by melatonin
  • During the night = the pineal gland secretes melatonin until dawn when there is an increase in light
  • In winter = less light mean secretion goes on for longer
  • This has a knock-on effect on the production of serotonin in the brain (low serotonin is linked to depressive symptoms)
• strength = menstrual cycle shows its evolutionary basis

  • For our distant ancestors = it may have been advantageous for females to menstruate together and become pregnant at the same time
  • in a social group = this would allow babies who had lost their mothers to have access to breast milk, thereby improving their chances of survival
  • Suggests that synchronisation is an adaptive strategy
• limitation = methodology used in synchronisation studies

  • There are many factors that may change a woman’s menstrual cycle and act as confounding variables (eg stress, changes in diet)
  • Any supposed pattern may occur by chance
  • This may be why other studies have not replicated Stern and McClintock’s original findings
  • Suggests that menstrual synchrony findings are flawed
• extra evaluation (SAD) = RWA

  • Light therapy (eg light box) reduces the debilitating effects of SAD (eg excessive sleepiness) in around 80% of people
  • It is also safer than using antidepressants
  • BUT = Rohan et al = recorded a relapse rate of 46% over successive winters, compared to 27% in a comparison group receiving CBT
  • Suggests that light therapy may be an effective short term treatment but additional treatments may be required if benefits are to be maintained