Biological Rhythms: Infradian Rhythms

Infradian Rhythms definition

A type of biological rhythm with a frequency of less than one cycle in 24 hours, such as menstruation and seasonal affective disorder.

The Menstrual Cycle

Female menstrual cycle is governed by monthly changes in hormone levels which regulate ovulation. - Cycle refers to the time between the first day of a woman's period to the day before her next period. - Typical cycle is approximately 28 days to complete (24-35 days is generally considered normal). - 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 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).

Synchronising the menstrual cycle

Although the menstrual cycle is an endogenous system, evidence suggests that it may be influenced by exogenous factors, such as the cycles of other women. - A study by Kathleen Stern and Martha McClintock, 1998, demonstrated how menstrual cycles may synchronise as a result of the influence of female pheromones.

Synchronising the menstrual cycle - Stern and McClintock, 1998

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 cycles, via a cotton pad placed in their armpit. - The pads were worn for at least 8 hours to ensure that pheromones were picked up. - The pads were treated with alcohol and frozen, to be rubbed on the upper lip of the other participants. - 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 second day of the cycle, and so on. - Stern and McClintock found that 68% of women experienced changes to their cycle which brought 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 of onset, and is described and diagnosed as a mental disorder in DSM-5. - As with other forms of depression, the main symptoms of SAD are persistent low mood alongside a general lack of activity and interest in life. - SAD is often referred to as the winter blues as the symptoms are triggered during the winter months when the number of daylight hours becomes shorter.

Seasonal Affective Disorder - Rhythm?

SAD is a particular type of infradian rhythm called a circannual rhythm as it is subject to a yearly cycle. - However, it can also be classed as a circadian rhythm as the experience of SAD may be due to the disruption of the sleep/wake cycle and this can be attributed to prolonged periods of daily darkness during winter.

Seasonal Affective Disorder - Melatonin

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 in the brain - a chemical that has been linked to the onset of depressive symptoms.

Strength

One strength of menstrual synchrony is that it may be explained by natural selection. Synchronisation of the female menstrual cycle, of the kind observed in the Stern and McClintock study, is thought by some to have evolutionary value. 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 during or after childbirth to have access to breast milk, thereby improving their chances of survival. This suggests that synchronisation is an adaptive strategy.

Weakness

One limitation of synchronisation studies is their methodological shortcomings. There are many factors that may affect change in a woman's menstrual cycle, including stress, changes in diet, exercise, etc. These may act as confounding variables, which means that any supposed pattern of synchronisation is no more than would have been expected to occur by chance. This may explain why other studies (e.g. Trevathan et al., 1993) have failed to replicate the findings. This suggests that menstrual synchrony studies are flawed.

Strength

One of the most effective treatments for SAD is light therapy, a box which simulates very strong light to rest the body's internal clock. Studies show this helps reduce the effects of SAD in about 80% of people (Sanassi, 2014). Light therapy is also preferred over antidepressants to treat SAD because it is regarded as safe. That said, light therapy can produce headaches and eye strain. Perhaps more tellingly, Kelly Rohan et al., 2009 recorded a relapse rate of 46% over successive wintersm compared to 27% in a comparison group receiving CBT.