Biological Rhythms (Infradian Rhythms)

What are infradian rhythms?

Type of biological rhythm with a frequency of less than one cycle in 24 hours 

E.g. menstruation or SAD

What is the menstrual cycle?

The cycle takes around 28 days to complete (Refers to the the first day of a woman’s period to the day before her next period)

During each cycle, rising levels of oestrogen cause the ovary to develop an egg and release it (ovulation).

After ovulation, 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 (the menstrual flow)

What was the aim of Stern and McClintock’s study?

To see if the menstrual cycle (endogenous system) can be influenced by exogenous factors (pheromones)

What was the procedure of Stern and McClintock’s study?

Demonstrated how menstrual cycles may synchronise as a result of the influence of pheromones (exogenous factors)

• Studied 29 women with a history of irregular periods.

• Samples of pheromones were gathered from 9 of the women at different stages of their menstrual cycle via a cotton pad in their armpit

• The pads were worn for 8 hours to ensure that the pheromones were picked up

• The pads were treated with alcohol and frozen, to be rubbed on the upper lip of other pps

• On day 1, pads from the start of the menstrual cycle were applied to all 20 women

• On day 2, they were all given a pad from the 2nd day of the cycle and so on

What were the findings of Stern and McClintock’s study?

68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’

What is seasonal affective disorder?

Depressive disorder which has a seasonal pattern of onset, and is described as a mental disorder in DSM-5.

What are the symptoms of SAD?

• Persistent low mood

• Lack of activity and interest in life

• Symptoms are triggered during the winter months when daylight hours become shorter

What type of rhythm is SAD?

Particular type of infradian rhythm called circannual rhythm (as its subject to a yearly cycle)

Can be classed of circadian rhythm as SAD may be due to the disruption of the sleep/wake cycle due to more darkness in winter

What is the psychology behind SAD?

During winter, lack of light in the morning means the secretin of melatonin continues for longer.  

This has an effect on serotonin levels in the brain (chemical linked to the onset of depressive symptoms)

What is a strength of research into infradian rhythms?

Menstrual synchrony research may be explained by natural selection

Synchronisation of the menstrual cycle (as observed my S+M) is thought to have evolutionary value.

For ancestors, it is advantageous for women to menstruate together and become pregnant at the same time → This allows babies who has lost their mothers during childbirth to have access to breast milk and improve their survival chances

This suggests that synchronisation is an adaptive strategy

What is a limitation of research into infradian rhythms?

Methodological limitations

There are many factors that may affect change to the menstrual cycle e.g. stress, changes in diet, exercise

These may act as confounding variables. 

This may explain why other studies (e.g. Trevathan et al) have failed to replicate these findings.

This suggests that menstrual synchrony studies are flawed.