infradian rhythms

The menstrual cycle

• The menstrual cycle is an example of infradian rhythm. It is covered by mentally changes in hormone levels that regulate ovulation.

• The typical cycle takes approximately 28 days to complete. It starts on the first day of a woman’s period and ends the day before her next.

• During each cycle rising levels of oestrogen caused to ovary to develop an egg and release it (ovulation)

• After ovulation the hormone progesterone helps the womb lining to grow thicker, readying the womb for pregnancy

• If pregnancy does not occur, the egg is absorbed into the body, the womb lining comes away and leaves the body (menstrual flow)

Synchronising the menstrual cycle

Although the menstrual cycle is an endogenous system evidence suggest that it may be influenced by exogenous factors such as the cycles of other women

• A study by Stern and McClintok demonstrated how menstrual cycles may synchronise as a result of the influence of hormones

• 29 women with a history of irregular periods

• Samples of pheromones were gathered from none of the women at different stages of their menstrual cycle, via cotton pad placed in their armpit - the pads were worn for at least eight hours to ensure pheromones were picked up

• Pads were treated with alcohol and were frozen, then were rubbed on the upper of other participants

• On day one, pads from the start of the menstrual cycle were applied to all 20 women and so on

• Stern and McClintok found that 68% of women experience changes to their cycle bringing them closer to the cycle of their ‘odour donor’

Seasonal affective disorder

Seasonal Affective Disorder (SAD) is a depressive disorder which has a seasonal pattern on onset - its main symptoms are low mood alongside a general lack of activity and interest in life

• SAD is often referred to as winter blues as the symptoms are triggered during the winter months when the number of daylight hours become shorter

• Psychologists have hypothesised that the hormone melatonin is implicated in the cause of SAD

• During the night , the pineal gland secretes melatonin until dawn when there is an increase in light - during winter, the lack of light in the morning means this secretion process continues for longer. This is thought to have a knock on effect on the production of serotonin (linked to depressive symptoms)

Strengths

• one strength of menstrual synchrony is that it may be explained by natural selection - synchronisation if the menstrual cycle is thought to have evolutionary value. For our distant ancestors it may have been advantageous for women to menstruate together and become pregnant at the same time. In a social group, this allows babies who have lost their mothers to still have access to breast milk. This suggests that synchronisation is an adaptive strategy

• One strength is there are effective treatments for SAD - light therapy is when a box stimulates very strong light that resets the body’s internal clock. Studies show that this helps reduce the effects of SAD in about 80% of people (Sanassi 2014)

Limitation

One limitation of synchronisation studies is their methodological shortcomings - there are many facts that may affect change to the menstrual cycle, including: stress , changes is diet , exercise … These act as confounding variables , which may be flawed and hard to replicate