Infradian Rhythms

• Infradian rhythms are biological rhythms with a frequency of less than one cycle in 24 hours (can be weekly/ monthly/ yearly)
• Menstrual cycles occur in females about every 28 days, endogenous control by the hormone's oestrogen and progesterone
• An infradian rhythm is a type of biological rhythm with a period longer than the period of circadian rhythm as menstruation and seasonal affective disorder (SAD)
• The menstrual cycle occurs in females about every 28 days, endogenous control by the hormone's oestrogen and progesterone
• Seasonal affective disorder (SAD) is a type of depression associated with seasonal changes, usually the onset of winter and increased darkness

  Symptoms include low mood, general lack of activity and lack of interest in life
• Menstrual Cycle:

  • Monthly infradian rhythm regulated by hormones either promoting ovulation or stimulating the uterus for fertilisation
  • Cycle refers to the time between the first day of the woman's period when the womb lining shed before her next period
• Menstrual Cycle:

  • During ovulation, rising levels of oestrogen cause ovaries to develop and egg and release it (ovulation)
  • After ovulation, progesterone levels increase in preparation for possible implantation of embryo into uterus; lining thickens
  • No pregnancy= body absorbs egg, womb lining sheds and leaves body
  • Although menstrual cycle is endogenous, research suggests it is influenced by exogenous factors like cycles of other women
• Influence of pheromones (McClinktock & Stern, 1998):

  • Demonstrating menstrual cycles synchronising as a result of female pheromones

  Method: 29 women with irregular period, sample of pheromones gathered from 9 of the women at different cycle stages; via cotton pad placed in their armpit worn for 8 hours, treated with alcohol and frozen and rubbed on upper lip of other 20 female participants

  Results: 68% of the women experienced changes to their cycle which brought them closer to the cycle of their 'odour donor'
• Seasonal affective disorder (SAD):

  • During the night, pineal gland produces more melatonin which induces sleep until dawn
  • In winter months, lack of light in the morning means secretion process continues for longer
  • Knock-on effect on production of serotonin in the brain
• S- Evolutionary basis (IR):

  Evolutionary psychologists claim synchronised menstrual cycles provide an evolutionary advantage for groups of women. Synchronisation of pregnancies mean childcare can be shared among multiple mothers who have children at the same time. Collective childcare ultimately increases chance of offspring survival
• W- Methodological limitations (IR):

  Criticisms made of early synchronisation studies and methods used. Many other factors may affect changes in a woman's menstrual cycle, including stress, changes in diet, exercise, etc. that might act on confounding variables. Research involved only small sample of women and relies on self-report. Synchronisation patterns may have occurred by change; other studies like Trevathan et al. (1993) failed to find any evidence of menstrual synchrony in an all female sample
• S- Role of melatonin in SAD(IR):

  Terman (1988) found that the rate of SAD is more common in Northern countries where the winter nights are longer. Terman found SAD affects 10% of people in New Hampshire and only 2% of residents in southern Florida. SAD is in part affected by light (exogenous zeitgeber) that results in increased levels of melatonin