Infradian rhythm

• What is infradian rhythm?


  A biological process that repeats less frequently than once every 24 hours (e.g. week or months)
• Example of infradian rhythm?

  Menstrual cycle
• Another example of infradian rhythm is seasonal affective disorder (SAD). This is an infradian rhythm that takes a year to complete. Emotional mood lowers in winter and improves in summer. It is suspected this is due to the lack of bright sunlight in winter
• The menstrual cycle is a 28 day cycle with a 6 day period of fertility
• What acts as endogenous pacemakers during the metrical cycle?


  Hormones (oestrogen and progesterone)
• Who conducted a study that investigated synchronisation of periods and link to pheromones?

  Stern and McClintock
• Procedure of Stern and McClintock’s study?

  20 women were given pads to wipe on their tip lip each day. The pads had been taken from the armpits of 9 donor women at varying stages of their menstrual cycle
• Findings of Stern and McClintock’s study?

  Women would either shorten or extend their menstrual cycle to match the donor, depending on when in the donor woman’s menstrual cycle the pad had been collected
• Conclusions from Stern and McClintock’s study?

  Synchronisation due to the presence of pheromones acting as an exogenous zeitgeber, an external cue for the timing of the infradian rhythm
• Although there have been some replications of McClintock with positive results, there have been a number showing no synchronisation, including Trevathan (1993) showing cohabiting lesbian couples did not synchronise
• As menstrual cycles can vary in length, women can appear to synchronise just due to their variability in their cycles, something not taken to consideration in all the studies showing positive results
• Evolutionary theories are conflicting. Suggesting synchronisation is advantageous, a way to stop one male dominating a group and having all the children, being unable to impregnate all women at once due to synchronisation, leading to genetic diversity in small human groups. Or maladaptive, due to women wanting to be fertilised by the male with the best genes, and needing to compete with other women for him if they synchronised
• Why might SAD might also be classed as and why?

  Circadian rhythm: could be due to a distraction of the sleep wake cycle
• What have psychologists hypothesised as the cause of SAD?

  Melatonin. During the winter, the lack of light in the morning means the pineal gland continues o secrete melatonin. This has a knock on effect on the production of serotonin
• One strength of menstrual synchrony research is that it may be explained by natural selection

  Synchronisation of the menstrual cycle may have evolutionary value. For our distant ancestors, it may have been advantageous for women to menstruate together and become pregnant at the same time so that babies can be cared for in a social group. Suggests synchronisation is an adaptive strategy
• Synchronisation studies have methodological limitations. There are many confounding variables like stress, changes in diet, exercise etc.
• Understanding of SAD has real world application for treatment

  Light therapy, a box which simulates cyber strong light reset the body’s internal clock. It has shown to reduce effects of SAD in about 80 % of people