Infradian rhythms

• infradian rhythms are biological rhythms that have a duration that is greater than 24 hours and may last days, weeks, months or may even be annual
• Stern and McClintock (1998) demonstrated how menstrual cycles may synchronise as a result of female pheromones on a sample of 29women. samples of pheromone were gathered from 9 of the women at different stages of their menstrual cycles, via a cotton pad in the armpit. the pads were treated with alcohol and frozen, to be applied to the upper lip of the other participants
• on day one, pads from the start of the menstrual cycle were applied to all 20 women and on day two, they were given a pad from the second day of the cycle. it was found that 68% of women experienced changes in their cycle that brought them closer to the cycle times of 'odour donor'
• however, it can be argued that there are many flaws in studies of synchronisation of women's menstrual cycles. there are multiple factors that impact on a menstrual cycle, including stress, of changes in diet. all of these external factors could become confounding variables
• from an evolutionary perspective, it may have been advantageous for females to menstruate together and therefore fall pregnant around the same time. new-borns could be cared for collectively within a social group increasing the chances of survival. however, Schank (2004) has questioned the validity of this theory as synchronisation would cause competition between the highest quality males and therefore avoidance would be preferable
• seasonal affective disorder is a depression based disorder which has a seasonal pattern of onset and is diagnosed as a mental disorder in the DSM-V. SAD symptoms are persistent low mood alongside a general lack of activity and interest in life. triggered in the winter months when the number of daylight hours reduces
• a strength of our understanding of SAD as an infradian rhythm is the practical application it offers. Eastman et al (1998) investigated the impact of phototherapy on SAD. phototherapy involves a light box that simulates strong light in the morning and evening, it's thought to reset melatonin levels in people with SAD. it was found that symptoms were reduces in 60% of sufferers
• however, they also reported a placebo effect of 30% in a control group treated with a 'sham negative ion generator'. due to the placebo effect it may be hard to conclude whether the drug or just the expectations of treatment has led to the improvement of symptoms