infradian rhythms

• Infradian rhythm

  A biological rhythm that takes longer than 24 hours to complete one cycle. One example is seasonal affective disorder. This is when emotional mood lowers in winter and improves in summer, and it is suspected this is due to the lack of sunlight in winter.
• Menstrual cycle


  28-day cycle with 6-days of fertility, however, cycles can be shorter or longer, ranging from between 24 and 35 days. Biological aspects of this cycle include ovulation and menstruation. These biological changes are regulated by the hormones oestrogen pre-ovulation and progesterone post-ovulation. The levels of these hormones act as endogenous pacemakers keeping the biological processes to time, acting as an internal body clock for the menstrual cycle.
• + Supporting evidence for menstrual cycle governed by exogenous zietgebers (external factors)


  E - Reinberg: examined a woman who spent three months in a cave with only a small lamp to provide light. Reinberg noted that her menstrual cycle shortened from the usual 28 days to 25.7 days.

  E - This result suggests that the lack of light (an exogenous zeitgeber) in the cave affected her menstrual cycle.

  L - Therefore, this demonstrates the effect of external factors on infradian rhythms.
• + Practical applications for SAD


  E - Research into SAD has produced beneficial practical applications for sufferers. One of the most effective treatments is phototherapy. This is a lightbox that stimulates very strong light in the morning and evening, thought to reset melatonin levels.

  E - This relieves symptoms in up to 60% of sufferers.

  L - Therefore, this makes life more 'normal' and bearable for sufferers.
• + Supporting evidence


  E - McClintock found 135 women who lived in the same dorm at university appeared to synchronise their cycles with their friendship groups.

  E - This suggests the menstrual cycle and infradian rhythm sychronise.

  L - However, as menstrual cycles can vary in length, women can appear to synchronise just due to their variability in cycles, which is not considered.