Infradian rhythms

• Infradian rhythms


  A type of biological rhythm with a frequency of less than one cycle in 24 hours - can be weekly, monthly, yearly etc...

  
  

  For example: menstrual cycles, seasonal affective disorder.

  
  

  These rhythms are engrained by endogenous pacemakers and exogenous zeitgebers.
• The female menstrual cycle

  The process of ovulation and menstruation - regulated by hormones which either promote ovulation or stimulate the uterus for fertilisation.

  
  

  Ovulation occurs roughly halfway through the cycle - when oestrogen levels are at their highest.

  
  

  After the ovulatory phrase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus.

  
  

  A typical cycle takes approximately 28 days to complete - can vary based on sleep, weight, diet, medication etc...
• Menstrual Cycle - EP's and EZ's

  EP = hormones: oestrogen and progesterone

  
  

  oestrogen = repairs and thickens uterus lining to prepare it for pregnancy

  progesterone = maintains uterus lining ready for the implantation of a fertilised egg

  
  

  they both inhibit LH so only one egg is released per menstrual cycle

  
  

  EZ = daylight/pheromones
• SAD - Seasonal Affective Disorder


  SAD is an infradian rhythm that is governed by a yearly cycle.

  
  

  A form of depression in which people experience depressive episodes (involving low mood and energy) during specific times of the year.

  
  

  Normally triggered during the winter months when the daylight hours are shorter - psychologists claim that melatonin is partly responsible.

  
  

  A type of infradian rhythm called a circannual rhythm as it's subject to occur every year.
• SAD - EP's and EZ's

  EP's = lack of morning light in winter means the pineal gland secrets melatonin for longer - has a knock-on effect on the production of serotonin in the brain - which has been linked to depressive symptoms.

  
  

  EZ's = light
• Stern and McClintock (1998)


  Stern and McClintock performed a lab experiment involving 29 women who all had irregular cycles.

  
  

  They wanted to demonstrate how menstrual cycles can synchronise as a result of the influence of female pheromones.
• Stern and McClintock (1998) - method

  9 women gave samples of their pheromones at different stages of their cycle via a cotton pad placed under their armpit.

  
  

  The pads were treated with alcohol, frozen and then rubbed on the upper lip of the other participants.

  
  

  This procedure continued throughout the menstrual cycle.
• Stern and McClintock (1998) - findings

  68% of women experienced changes to their cycle which brought them closer to the cycle of their 'odour donor' (whose pheromones they were exposed to).
• What does this study suggest?

  The synchronisation of menstrual cycles can be affected by pheromones which have an effect on the bodies of people close by.

  
  

  Like how hormones have an effect on the body of the person producing them.

  
  

  Indicate that external factors must be taken into consideration when investigating infradian rhythms - more holistic approach should be taken.
• Evolutionary advantage


  Menstrual synchrony is evolutionarily advantageous - females would fall pregnant at the same time meaning new-borns could be cared for collectively. This increased chances of survival.
• A03 - Limitation with McClintock's study (P)

  Critics argue that many factors may affect change in a woman's menstrual cycle - including stress, changes in diet and exercise.
• A03 - Limitation with McClintock's study (E)

  The validity of the results from McClintock's study are questioned - as they didn't control for extraneous factors like smoking, physical activity, and alcohol consumption.
• A03 - Limitation with McClintock's study (E)

  This means that it's difficult to conclude that the pattern of synchronisation seen in Stern and McClintock's study - and doubts the influence of pheromones.
• A03 - Limitation with McClintock's study (L)

  This raises doubts about whether they overstated the strength of the influence of pheromones as an exogenous zeitgeber which can entrain infradian rhythms.

  
  

  Moreover, it was based on a small sample of 29 women (which limits the generalisability of findings), and relied on participant's self-report of the onset of their cycle.
• Phototherapy


  Light boxes that stimulate strong light in the morning and evening are thought to reset melatonin levels in SAD sufferers.
• A03 - Gloth et al (P)


  Gloth conducted a randomised, controlled trial in a group of 15 subjects with SAD.

  
  

  8 subjects received Vitamin D supplements, while the other 7 subjects received phototherapy.

  
  

  At the onset of treatment, and after 1 month of therapy, subjects were administered depression scales.
• A03 - Gloth et al (E)


  Gloth reported that 'all subjects receiving vitamin D improved in all outcome measures' - including a 74% improvement in their depression measures.

  
  

  However, the phototherapy group showed no significant change in depression scale measures.
• A03 - Gloth et al (E)


  Therefore, this suggests that phototherapy is ineffective in treating SAD, and that Vitamin D may be a more important treatment.
• A03 - Gloth et al (L)


  Since phototherapy involves exposure to bright light to reset melatonin levels, this implies that melatonin and serotonin levels have little parts to play in the development of SAD - but that low Vitamin D levels have a more significant role.