INFRADIAN RHYTHMS

• A type of biological rhythm which has a duration of over 24 hours, either weekly, monthly or annually. This includes the menstrual cycle and seasonal affective disorder (SAD)
• The menstrual cycle is an example of an infradian rhythm. It is governed by monthly changes in hormone levels which regulate ovulation. It lasts approximately 28 days from the first day of a period to the day before the next period. 
• Regulated by hormones (endogenous pacemaker): 

  1. Oestrogen increases to cause the ovary to develop an egg and release it (ovulation) 
  2. Progesterone thickens the womb lining which prepares the uterus for pregnancy 
• Exogenous zeitgebers that may affect the menstrual cycle include diet, exercise, hydration. 
• STERN AND MCCLINTOCK (1998)

  • Their aim was to investigate whether exogenous zeitgebers can influence the menstrual cycle and their sample was of 29 women with irregular periods. 
  • Samples of pheromones were taken from women at various stages of their menstrual cycle via a cotton pad placed under their armpit for 8 hours 
  • The pads were sterilised and rubbed on the upper lip of other women 
• Results of Stern and McClintock study

  • 68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’ 
  • This suggests exogenous zeitgebers – the cycle of other women – can lead to menstruation synchronisation 
• Seasonal Affective Disorder (SAD)


  A form of depression triggered during the winter in reduced daylight hours. Symptoms include persistent low mood, low interest in activities and life and changes in appetite. During winter, the lack of light means the melatonin secretion process occurs for longer periods of the day, also influencing the production of serotonin in the brain. 
• Strength 


  ✅Support evolutionary explanations – useful to our ancestors as it meant they could get pregnant and raise their children together, to improve chances of survival 
• Strength (2)

  ✅Understanding SAD has led to development of treatments in this case phototherapy. It is a lightbox that stimulates a very strong light in the morning and evenings and is thought to reset melatonin levels in people with SAD. By reducing melatonin, it will in turn increase serotonin levels, increasing a person's happiness and thus their quality of life. 
• Limitation

  ❌Confounding variables – many factors may influence a woman's menstrual cycle such as stress and exercise – no guarantee that the pheromones had a direct impact. Additionally, the researchers used self-report techniques – may not have been truthful – lacks internal validity.