infradian rhythms

• Infradian rhythm - a type of biological rhythm with a frequency of less than one cycle in 24 hours
• examples of infradian rhythms:

  • menstruation
  • seasonal affective disorder (SAD)
• the menstrual cycle is controlled by the endocrine system as changes in hormone levels regulate ovulation
• The menstrual cycle takes around 28 days to complete
• During the menstrual cycle, rising levels of oestrogen cause the ovary to develop and release an egg. After ovulation, the hormone progesterone helps the womb grow thicker, ready for pregnancy. If pregnancy doesn’t occur, the egg is absorbed into the body and the womb lining comes away
• It's thought the menstrual cycle responds to light and odour zeitgebers
• Stern and McClintock (1998) conducted a study involving 29 women with irregular periods.
• Stern and McClintock gathered samples of pheromones from 9 women at different stages of the menstrual cycle via a cotton pad placed in their armpits. The treated pads were rubbed on the top lip of the other pps. On day 1, pads from the start of the cycle were applied to the other 20 women, day 2 they were given a pad from day 2 of the cycle and so on
• Stern and McClintock found that 68% of women experienced changes to their cycle which brought them closer to their odour donor. This shows that the menstrual cycle of a woman can be altered by communication via pheromones
• Seasonal Affective Disorder (SAD) is a depressive disorder which has a seasonal pattern of onset
• Main symptoms of SAD are persistent low mood and lack of interest in life
• The trigger for SAD seems to be a decrease in daylight hours in winter
• It is thought the hormone melatonin might affect SAD. During the night the pineal gland secretes melatonin until dawn when the light increases. During the winter melatonin is secreted for longer, which is thought to affect serotonin in the brain
• McClintock’s study which suggests women may experience menstrual synchrony has evolutionary value
• In evolutionary terms, if females menstruated at a similar time then they may also fall pregnant together. This is a strength as this would mean newborns would be cared for collectively within a social group promoting increased chance of survival
• this evolutionary theory is questioned by Schank who argued that too many females cycling together would create competition for high ranking males and as such could lower the fitness of potential offspring which goes against the evolutionary theory
• Confounding variables such as stress, diet, exercise may affect a woman’s menstrual cycle. This means that the findings of McClintock’s study can be criticised and argued that the results are down to chance
• Other studies (Trevathan) have failed to find any synchrony between women. this decreases the validity of McClintock's study
• Most research into the role of pheromones has been conducted on animals. The conclusions may not be applicable to humans and so evidence for the effects in human behaviour remains speculative and inconclusive
• there is practical application of treatment for SAD. One of the most effective treatments for SAD is phototherapy, where a light box stimulates very strong light in the morning and evening. It’s thought to reset melatonin levels and relieves symptoms in up to 60% of sufferers (Eastman et al 1998)
• However the Eastman's study also recorded a placebo effect of 30% using a sham negative ion generator. This casts doubt on the chemical influence of phototherapy