infradian rhythms

• menstrual cycle

  • governed by monthly changes in hormone levels which regulate ovulation
  • cycle = 1st day womb shedding to day before next period (typically 28 days)
  • rising evils of oestrogen cause ovary to develop egg + release it
  • after ovulation: progesterone helps womb lining to grow thicker ready for pregnancy
• synchronising menstrual cycle

  • endogenous system influenced by exogenous factors (cycles of other women)
  • Stern + McClintock: 29 women with a history of irregular periods, samples of pheromones gathered from 9 women at different cycle stages, cotton pad worn in armpit for 8 hours then treated with alcohol + frozen + placed on upper lop of other woman, day 1 pads from day 1 donor applied to all 20 women etc - 68% women experienced changes to cycle that brought them closer to cycle of odour donor
• seasonal affective disorder (SAD)

  • depression with seasonal pattern of onset
  • symptons: persistent low mood alongside general lack of activity
  • triggered when daylight hours become shorter
  • type of infradian rhythm called circannual rhythm - subject to yearly cycle
  • can be classes as circadian rhythm as it may be due to disruption to sleep/wake cycle
  • hormone melatonin implicate in case of SAD: during night pineal gland secreates melatonin until dawn when there’s increase in light, winter means segregation process continues for longer (knock-on effect on production of serotonin in brain)
• evaluations

  • S: Real-world application: most effective treatment for seasonal depression is light therapy (box that stimulates strong light to reset the body’s internal clock) – Sanassi: helps reduce effects of SAD in 80% of people + more ‘safe’ than drug therapy HOWEVER light therapy can produce headaches + eye strain – Rohan et al: recorded relapse rate of 46% over successive winters, compared to 27% in comparison group receiving CBT
• evaluations

  • S: May be explained by natural selection: synchronisation of menstrual cycle thought to have evolutionary value, for distant ancestors it may have been advantageous for women to menstruate together = become pregnant at the same time (allow babies whose mothers died to receive breast milk form a different mother) - synchronisation an adaptive strategy
• evaluations

  • W: Methodological shortcomings: many factors may effect change to cycle (stress, changes in diet, exercise) – Trevathan et al: confounding variables, any supposed pattern of synchronisation is no more than would have been expected to occur by chance, may explain why other studies failed to replicate findings - menstrual synchrony studies are flawed