INFRADIAN RHYTHMS

~The Menstrual Cycle~

→ Female reproductive cycle and takes approximately 28 days to complete

→ The menstrual cycle is an endogenous system governed by the body’s internal body clock

→ During each cycle:

1. The pituitary gland releases FSH (follicle stimulating hormone) which causes an egg to mature in the ovary

2. This causes oestrogen to be released from the ovaries which causes the uterus lining to thicken

3. LH (lutenising hormone) is then released from the pituitary gland which triggers the release of the mature egg form the ovary (ovulation)

4. After ovulation, progesterone is released form the ovaries to maintain the uterus lining

5. If pregnancy does not occur, progesterone levels fall and the egg and uterus lining leave the body (menstrual flow)

~Seasonal Affective Disorder (SAD)~

→ A depressive disorder with a seasonal pattern of onset

→ The main symptoms of SAD are persistent low mood with a general lack of activity

→ These symptoms are triggered during the winter months when the number of daylight hours falls

→ Number of daylight hours is therefore an important exogenous zeitgeber in SAD

→ Psychologists have hypothesised that the hormone melatonin is implicated in SAD

→ During the night, when it’s dark, the pineal gland secretes melatonin until dawn when there is an increase in light

→ During the winter, the lack of light in the morning means that this secretion process continues for longer, which is thought to have a knock on effect on the production of the neurotransmitter serotonin in the brain (serotonin has been linked to depressive symptoms)

EVALUATION

Research Support

→ EVOLUTIONARY BASIS

• One strength of menstrual synchrony is that it may be explained by natural selection.

• Synchronisation of the menstrual cycle in a social group is thought by some to have an evolutionary value. For our distant ancestors, this may have been advantageous because newborns who were cared for collectively would increase their chances of survival.

• However, the validity of the evolutionary perspective has been questioned. Schank (2004) argued that too many females cycling together within a social group would produce competition for the highest-quality males (and therefore lower the fitness of potential offspring).

• From this point of view, the avoidance of synchrony would be the most adaptive evolutionary strategy and hence naturally selected.

→ REAL-WORLD APPLICATION

• One of the most effective treatments for seasonal affective disorder (SAD) is light therapy, a box which stimulates very strong light in the body’s internal clock.

• Studies show this helps reduce symptoms of go to 60% of sufferers (Eastman et al, 1998).

• However, the same studies record a relapse rate of 46% over successive winters, meaning some must return for more light therapy treatments.

• This is good for the economy as studies indicate it may reduce reliance on pharmacological treatments (e.g. antidepressants)

Conflicting Evidence

→ METHODOLOGICAL LIMITATIONS

• The limitation of synchronisation studies is the methodological shortcomings.

• The timing of cycle onset (e.g. when it started) can be affected by confounding variables, including stress, changes in diet, exercise etc.

• These may act as confounding variables, which means that any apparent pattern of synchronisation is no more than what would have been expected by chance. This may explain why other studies have failed to replicate the findings.

• This suggests that menstrual synchrony studies are flawed.