Infradian Rhythms

• (AO1) Define infradian rhythms + give example

  The menstrual cycle is an example of an IR, as it only occurs les then every 24 hours, lasting on average 28 days.

  
  

  It begins on day one, with the start of a period.

  
  

  Around day 12, LH hormones rise, leading to ovulation.

  
  

  After this, there is an increase in progesterone, which can lead to uncomfortable side effects.

  
  

  If fertilisation does not occur, progesterone levels fall, and the cycle begins again.

  
  

  Despite being controlled by the endocrine system, zeitgebers can also impact this rhythm.

  
  

  This biological rhythm continues in the female body until menopause.
• (AO3) Attraction

  Point: IR like the menstrual cycle may have an impact on behaviour, particularly in terms of attraction.

  
  

  Evidence: Research suggests that changes occur depending on where a person is in the cycle. During the fertile phase (shortly after ovulation), individuals are more likely to be attracted to masculine characteristics, even if they are usually attracted to women only. In contrast, during the least fertile phase, more feminine characteristics are found to be attractive.

  
  

  Evaluate: Useful in explaining behaviours linked to human mating preferences, showing how biological rhythms like the menstrual cycle can influence attraction & partner choice.
• (AO3) Pheromones

  Point: Some research suggests synching of menstrual cycles may occur due to pheromones.

  Evidence: There is some evidence that women living together may synchronise, such as research where sweat was applied to the lip, leading to reported synchronised cycles.

  
  

  Evaluate: However, the study had a small sample, and findings may reflect subjective validation (tendency to interpret info in way that aligns with beliefs), questioning how meaningful synchronisation is in real terms.
• (AO3) S - SAD

  Point: IR provide support for melatonin in SAD (seasonal affective disorder).

  
  

  Evidence: Research shows that SAD is more common in northern countries where winter nights are longer. E.g. SAD effects 10% of people in New Hampshire (northern part of US) and only 2% in Florida.

  
  

  Evaluate: Suggests exposure to light (an EZ) impacts melatonin production, with longer periods of darkness leading to higher melatonin levels, which may contribute to onset of SAD .