Biological rhythms: Infradian and ultradian


Infradian Rhythms (IR)

  • 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)
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Research study

  • 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. 
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Seasonal Affective Disorder (SAD)

  • 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 
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Ultradian Rhythms

  • 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. 
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Evaluation of biological rhythms

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
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Evaluation of biological rhythms

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
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Evaluation of biological rhythms

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
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Evaluation of biological rhythms

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
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