Biological rhythms: Infradian and ultradian rhythms

Question 1

what is an infradian rhythm

Answer 1

A type of biological rhythm with a frequency of less than one cycle in 24 hours. Such as the menstrual cycle and seasonal affective disorder. The menstrual cycle is governed by monthly changes in hormone levels which regulate ovulation. The menstrual cycle typically takes 28 days to complete.

Question 2

Synchronising the menstrual cycle

Answer 2

The menstrual cycle is an endogenous system however evidence suggests it may be affected by exogenous factors, such as cycles of other women. Stern and McClintock studied 29 women. Samples of pheromones were gathered from 9 women at different stages of their menstrual cycle and collected on pads. The pads were treated and then rubbed on upper lip of other participants. Stern and McClintock found that 68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’.

Question 3

Seasonal affective disorders

Answer 3

SAD is a depressive disorder which has a seasonal pattern of onset. 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. It’s been hypothesised that melatonin is implicated in the cause of SAD. Lack of light means melatonin is secreted for longer and thought to have an effect on the production of serotonin – a chemical linked to the onset of depressive symptoms.

Question 4

Evaluation of infradian rhythms

Answer 4

strength - synchronisation of the menstrual cycle is thought to have evolutionary value. It may have been advantageous for women to menstruate together and become pregnant at the same time. This would allow babies who lost their mother during or after childbirth to have access to breast milk, thereby increasing their chances of survival. Therefore, this suggests that synchronisation is an adaptive strategy.

Weakness – synchronisation studies have methodological issues. Many factors effect changes to the menstrual cycle, including stress, changes in diet, exercise etc. These may act as confounding variables explaining why other studies, such as Trevathan et al 1993, have failed to replicate the findings. This suggests that menstrual synchrony studies are flawed.

Question 5

what is an ultradian rhythm

Answer 5

A type of biological rhythm with a frequency of more than one cycle in 24 hours such as the stages of sleep.

Question 6

stages of sleep

Answer 6

sychologists have identified 5 stages of sleep that altogether span around 90 minutes. Each of the stages is characterised by differing levels of brainwave activity that can be monitored by EEGs.

Stages 1 and 2 – light sleep. In stage 1, brainwaves are high frequency and have short amplitude (alpha waves). In stage 2, alpha waves continue but have random sleep spindles.

Stages 3 and 4 – deep sleep. Brain waves are delta waves with lower frequency and higher amplitude.

Stage 5 (REM sleep) - brain produces theta waves and eyes occasionally move around. Dreams are most often experienced here.

Question 7

Evaluation of ultradian rhythms

Answer 7

Weakness - the way in which such research is conducted may tell us little about ultradian rhythms in humans. When investigating sleep patterns, participants are subjected to a specific level of control and are attached to monitors that measure such rhythms. This may be invasive for the participant, leading them to sleep in a way that does not represent their ordinary sleep cycle. This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.

Weakness – there is significant variation of ultradian rhythms between people. Tucker et al (2007) found large differences in the duration of each sleep stage, particularly stages 3 and 4. tucker et al suggests these differences are likely to be biologically determined. This suggests that there must be innate individual differences in ultradian rhythms