Lesson 13 - Infradian rhythms

Question 1

What are infradian rhythms?

Answer 1

Cycles that last longer than 24 hours

Question 2

The menstrual cycle

Answer 2

• Regulated by hormones and promote ovulation or stimulate the uterus for fertilisation.
• Generally lasts for 28 days, but can range from 23-36
• While it is mostly influenced by internal factors like hormones, some evidence claims it can be influenced by external factors, like other women’s cycles

Question 3

Strengths of the menstrual cycle

Answer 3

• Reinberg (1967) found evidence of exogenous zeitgebers that affect the cycle. Reinberg studied a woman who spent 3 months in a cave with only a small lamp. Her menstrual cycle shortened from 28 days to 25.7 days. The lack of light may have been the cause of this.
• Russell et al (1980) found evidence to suggest that menstrual cycles can become synchronised to other women’s cycles through odour exposure. Sweat samples of one group of women were rubbed onto the upper lips of another group of women. Despite being fully separate, their cycles synchronised. It is suggested that pheromones are an exogenous zeitgeber for the menstrual cycle.
• Penton-Volk found that infradian cycles like the menstrual cycle can regulate behaviours as well. Women expressed preferences for more feminine faces at their least fertile stage, and attracted to more masculine faces at their most fertile stage.

Question 4

Weaknesses of the menstrual cycle

Answer 4

• Confounding variables may weaken the causal relationship in synchronisation studies. Stress, changes in diet and exercise are all contributing factors. Synchronisation may have been completely due to chance, which weakens the suggestion that the menstrual cycle is an infradian rhythm. Studies have failed to replicate it as well.
• Most research seems to place more of an emphasis on exogenous zeitgebers rather than endogenous ones, therefore it can be considered environmentally deterministic

Question 5

Seasonal affective disorder (SAD)

Answer 5

Research has found seasonal variations in mood, with some people becoming depressed in the winter. SAD is a yearly infradian cycle.

Some researchers place the cause on melatonin, with the lack of light during winter as a trigger for longer periods of melatonin release. This may cause depressive symptoms.

Question 6

Strengths of SAD

Answer 6

• Terman (1988) found that people living in New Hampshire in the north of the US, roughly 10% of people suffer from SAD, and this could be because of the longer winter nights that occur in northern countries. It only affects 2% of residents in southern Florida. This is evidence that light is an exogenous zeitgeber for SAD.
• There are also practical applications. Eastman et al (1998) found that 60% of sufferers of SAD had improves symptoms when treated with phototherapy, a light box that emits lots of light every morning and evening. It is thought to reset melatonin levels and increase serotonin. This is further evidence of SAD being primarily caused by low light levels

Question 7

Weaknesses of SAD

Answer 7

Eastman’s study was carried out again but it found that 30% of participants showed improvements when treated with a pretend ‘negative ion generator’. Therefore phototherapy may have been a placebo effect, and not really effective.