biological rhythms: infradian rhythms

• the human female menstrual cycle is an example of an infradian rhythm and it is 28 days long. Rising levels of oestrogen cause the ovary to release an egg (ovulation). Then progesterone helps the womb lining to thicken, readying the womb for pregnancy. If pregnancy does't occur, the egg is absorbed and the womb lining comes away (menstrual flow)
• Stern and Mclintock studied 29 women with irregular periods. Pheromones were taken from some at different stages of their cycles, via a cotton pad under their armpits. These pads were cleaned with alcohol and later rubbed on the upper lips of the other participants. 68% of women experienced changes to their cycle which brought them closer to the cycle of their 'odour donor'
• seasonal affected disorder is a depressive disorder (resulting in low mood, lack of activity) with a seasonal pattern. It is often called 'winter blues' because during the winter months the number of daylight hours becomes shorter. It is another example of infradian rhythms
• during the night, the pineal gland secretes melatonin until dawn when there is an increase in light. In winter, less light means secretion goes on for longer. This has a knock on effect on the production of serotonin in the brain (low serotonin is linked to depressive symptoms)
• One strength of infradian rhythms is the research on menstrual cycles shows its evolutionary basis. For our distant ancestors it may have been advantageous for females to menstruate together and become pregnant at the same time. In a social group, this would allow babies who had lost their mothers to have access to breast milk, thereby improving their chances of survival. This suggests that synchronisation is an adaptive strategy
• One limitation of infradian rhythms is the methodology used in synchronisation studies. There are many factors that may change a women's menstrual cycle and act as confounding variables like stress and changes in diet. Any supposed pattern may occur by chance. This may be why other studies have not replicated Stern and McClintock's original findings. This suggests that menstrual synchrony studies are flawed