Infradian and ultradian rhythms

Female menstrual cycle

The human female menstrual cycle is about 28 days (i.e. less that one cycle in 24 hours - infra diem meaning 'below a day'). 

Raising levels of oestrogen cause the ovary to develop and release an egg (ovulation). Then progesterone helps the womb lining to thicken, readying the body for pregnancy. If pregnancy doesn't occur then the egg is absorbed into the body and the womb lining comes away and leaves the body (menstrual flow). 

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Stern and McClintock (1998)

Studied 29 women with irregular periods.

Pheromones were taken from some at different stages of their menstrual cycles via a cotton pad under their armpits. These pads were cleaned with alcohol and later rubbed on the upper lips of the other participants on their corresponding cycle day. 

68% of women experienced changes in their cycle which brought them closer to that of their 'odour donor'. 

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Strength - Evolutionary value

P - One strength of research on the menstrual cycle is that it shows evolutionary value. 

E - It may have been advantageous for our ancestor females to menstruate together and become pregnant around the same time, so offspring could be cared for collectively, increasing their chances of survival. 

CA - However, Schank (2004) questions the validity of this perspective - too mant females cycling together within a social group would produce competition for highest quality males (lowering the fitness of any potential offspring). From this point of view, the avoidance of synchrony would appear to be the most adpative evolutionary strategy and therefore, the one that would be naturally selected. 

E - On the other hand, there is evidence to show that females' mentrual cycles are able to synchronise somewhat due to the pheromones released when at various stages of the cycle. 

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Limitation - Methodology

P - One limitation is the methodology used in synchronisation studies. 

E - Commentators argue that there are many factors that may change a women's menstrual cycle and act as a confounding variable in research (e.g. strees, changes in diet). So any pattern of synchronisation (e.g. in Stern and McClintock's study) is what we would expect to occur by chance. Also, research involves small samples of women and relies on them self-reporting the onset of their own cycle (may be innacurate). 

E - This suggests that important aspects of synchronisation studies may lack validity. 

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Sleep stages

Stages of sleep are an ultradian rhythm. 

Sleep pattern occurs in 90 minute periods, divided into 5 stages that are each characterised by a different level of brainwave activity (monitored using EEG). 

Stages 1 and 2: light sleep. Person may be easily woken. Brainwaves become slower and more rhythmic (alpha waves), slowing futher as sleep becomes deeper (beta waves). 

Stages 3 and 4: deep sleep or slow wave sleep. Difficult to rouse someone. Characterised by delta waves which are slower still and have great amplitude. 

Stage 5: REM (rapid eye movement). Fast, jerky activity of eyes. Body paralysed yet brain activity speeds up. 

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Strength - supporting evidence

P - One strength is evidence supports qualitatively different stages of sleep. 

E - A landmark study by Dement and Kleitman (1957) monitored the sleep patterns of 9 participants in a sleep lab and found evidence for stages of sleep, including REM. REM activity during sleep correlated with dreaming; brain activity varied according to how vivid the dreams were; particpants woken up during dreaming and reported accurate recall of their dreams. 

CA - However, the small sample size of 9 people means that it isn't representative of the wider population and can therefore, not be generalised. 

E - On the other hand, this study suggests that REM (dreaming) sleep is a distinct ultradian rhythm and an important component of the ultradian sleep cycle. 

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