Biological rhythms: Infradian rhythms and ultradian rhythms Flashcards
The female menstrual cycle is an infradian rhythm.
The human female menstrual cycle is about 28 days (i.e. less than one cycle in 24 hours - infra diem meaning ‘below’ a day).
Rising levels of oestrogen cause the ovary to release an egg (ovulation).
What happens next and if pregnancy doesn’t occur?
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
Then progesterone helps the womb lining to thicken, readying the womb for pregnancy.
If pregnancy does not occur, the egg is absorbed and the womb lining comes away (menstrual flow).
Exogenous zeitgebers may synchronise menstrual cycles.
Stern and McClintock (1998) studied 29 women with irregular periods.
Pheromones were taken from some at different stages of their cycles, via a cotton pad under their armpits.
What did they do with these pads?
What did they find?
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
These pads were cleaned with alcohol and later rubbed on the upper lips of the other participants.
68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’.
SAD is an infradan rhythm.
Seasonal affective disorder (SAD) is a depressive disorder (low mood, lack of activity) with a seasonal pattern.
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
Often called the ‘winter blues’ because the symptoms are triggered during the winter months when the number of daylight hours becomes shorter.
SAD may be caused by the hormone melatonin.
During the night, the pineal gland secretes melatonin until dawn when there is an increase in light.
In winter, less light means secretion goes on for longer.
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
This has a knock-on effect on the production of serotonin in the brain (low serotonin is linked to depressive symptoms).
Stages of sleep are an ultradian rhythm.
Sleep pattern occurs in 90-minute periods (i.e. more than one cycle in 24 hours - ultra diem meaning ‘beyond’ a day).
AO1: Ultradian rhythms (more than 1 cycle in 24 hours)
Divided into five stages, each characterised by a different level of brainwave activity (monitored using EEG).
5 stages of sleep have been identified.
Stages 1:
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
Light sleep where a person may be easily woken.
Brain waves are high frequency and have a short amplitude. These are alpha waves.
5 stages of sleep have been identified.
Stage 2:
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
The alpha waves continue but there are occasional random changes in pattern called sleep spindles.
5 stages of sleep have been identified.
Stages 3 and 4:
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
This is known as deep sleep or slow wave sleep (SWS).
The individual waves now have lower frequency and higher amplitude.
It is difficult to wake someone at this point.
5 stages of sleep have been identified.
Stages 5 (REM sleep):
AO1: Infradian rhythms (less than 1 cycle in 24 hours)
The body is paralysed yet brain activity closely resembles that of the awake brain.
During this time, the brain produces theta waves and the eyes occasionally move around, thus rapid eye movement (REM)
Dreams most often experienced during REM sleep, but may also occur in deep sleep.
One limitation is individual differences in sleep stages.
Tucker et al. (2007) found large differences between participants in the duration of stages 3 and 4.
They suggest that these differences are biologically determined.
This makes it difficult to describe ‘normal sleep’ in any meaningful way.
One strength is understanding age-related changes in sleep.
SWS reduces with age.
Growth hormone is produced during SWS so this becomes deficient in older people.
van Cauter et al. (2000) suggest the reduced sleep may explain impairments in old age.
SWS sleep can be improved using relaxation and medication.
This suggests that knowledge of ultradian rhythms has practical value.
Research on the menstrual cycle shows its evolutionary basis.
For our distant ancestors it may have been advantageous for females to menstruate together and become pregnant at the same.
In a social group, this would allow babies who had lost their mothers to have access to breast milk, thereby improving their chances of survival.
This suggests that synchronisation is an adaptive strategy.
