Biological rhythms: Infradian and ultradian

• IR's have that occur less than once everyday - for instance the menstrual cycle
• The menstrual cycle is the monthly changes in hormone levels which regulate ovulation
• The cycle refers to the time between the 1st day of a woman's period, when the womb lining is shed, to the day before her next period
• The typical cycle takes 28 days to complete on average

• During each cycle, rising levels of the hormone oestrogen cause the overy to develop an egg and release it (ovulation)
• After ovulation, the hormone progesterone helps the womb lining to grow thicker, readying the body for pregnancy
• If pregnancy does not occur = egg is absorbed into the body, the womb lining comes away and leaves the body (the menstrual flow)

• Evidence suggests that the menstrual cycle is influenced by external factors = exogenous factors such as the cycles of other women
• A study by Stern and McClintock shows how menstrual cycles may synchronise as a result of the influence of female pheromones
• Sample size = 29 women = had irregular periods
• Samples of their pheromones (from 9 of the p's) were placed in their armpits 
• Pads were worn for at least 8 hours
• Day one = pads from the start of menstrual cycle were applied to all 20 women
• Day two = they were all given a pad from the second day of the cycle etc
• They found that 68% of women experienced changes in their cycle which brought them closer to the cycle of their donor. 

• SAD = depressive disorder
• Symptoms = persistent low mood / lack of activity / interest in life
• SAD is referred to the winter blues as the symptoms are triggered during the winter months when the number of daylight hours becomes shorter
• SAD = is a type of infradian rhythm = circannual rhythm as it is subject to a yearly cycle
• SAD = can also be classed as a circadian rhythm because SAD may be due to the disruption of the sleep/wake cycle and this can be attributed to prolonged periods of dailt darkness during winter
• Hormone melatonin = cause of SAD
• At night - pineal gland secretes melatonin until dawn where there is an increase in sunlight
• During winter = lack of light in the morning means this secretion process continues for longer
• Melatonin = knock-on effect on the production of serotonin in the brain - a chemical linked to depressive symptoms such as depression 

• UR = stages of sleep - the sleep cycle 
• 5 stages of sleep that altogether span approximately 90 mins - a cycle that continues throughout the course of the night
• Each stage has a different level of brainwave activity which can be monitored using an EEG:
  • Stage 1 and 2: light sleep where the person may be easily woken / brainwave patterns start to become slower and more rhythmic (alpha waves), becoming slower as sleep becomes deeper (theta waves)
  • Stage 3 and 4: deep sleep = brainwave becomes slower which involve theta waves and there is greater amplitude than earlier wave patterns / difficult to wake someone up at this point
  • In stage 5, REM sleep: this stands for rapid eye movement - this when the body is paralysed yet brain activity speeds up. It's called REM sleep beciase of the fast, jerky activity of the eyes under the eyelids / REM sleep is correlated to dreaming. 

Evolutionary basis of the menstrual cycle

• McClintock study = evolutionary value
• For our ancestors it may be benefical for females to menstruate together and therefore fall pregnant around the same time
• This means new-borns could be cared for collectively within a social group increasing the chances of the offspring's survival
• Shank argues that if too may females were cycling together = produce competition for the highest quality males = lowering the fitness of any potential offspring = lacks validity
• Hence avoidance of synchrony = more adaptive evolutionary strategy = naturally selected

Methodlogical limitations in synchronisation studies

• There other factors that may change a woman's menstrual cycle, and this was not considered in the studies
• Factors such as stress, changes in diet, exercise etc these may act as confounding variables
• So in McClintock's study the pattern on sychronisation found may have occured by chance
• Small samples used and relies on p's self-reporting the onset of their own cycle = lacks representativeness / reliablity / validity / individual differences/ cultural differences
• Trevathan et al failed to find any evidence of menstrual synchrony in all female samples

Evidence supports the idea of distinct stages in sleep

• Dement and Kleitman study 
• They monitered the sleep patterns of 9 p's in a sleep lab = small sample / controlled setting
• Brainwave activity was recorded on an EEG
• Researchers controlled the effects of caffeine and alcohol
• Results: REM activity during sleep was highly correlated with dreaming / brain activity varied according to how vivid dreams were 
• P's woke = could accurately recall their dreams
• Replications of the study have found similar findings = reliable 
• Study suggests that REM sleep = important component of the ultradian sleep cycle

Animals studies 

• Knowledge of the effects of phermones on behaviour comes from animal studies
• Sea urchins release pheromones into the surrounding water so other urchins in the colony will eject their sex cells at the same time
• In contrast, the effects in human behaviour remains speculative and inconclusive

Practical application - SAD

• Effective treatments is phototherapy 
• A lightbox that stimulates very strong light in the morning and evening = reset melatonin levels in people with SAD
• Relieves symptoms up to 60% of sufferers
• Same study recorded a placebo efect of 30% using a 'sham negative-ion generator' = p's were told it was another form of treatment = casts doubt on the chemical influence of phototherapy