Biological Rhythms

• Biological rhythms refer to the cyclical changes in the way our bodily systems behave.
• Circadian rhythms- have a periodicity of about 24 hours (sleep-wake cycle).
• Ultradian rhythms- have a periodicity of less than 24 hours (the sleep cycle).
• Infradian rhythms- have a periodicity of more than 24 hours (the menstrual cycle).
• Our biological rhythms are coordinated in 2 ways:

  1. Endogenous pacemakers are our internal biological 'body clocks'- mechanisms within the body that govern biological rhythms.
  2. Exogenous zeitgebers are external environmental cues, eg external factors that help to regulate or 'set' the endogenous pacemakers.
• Circadian rhythms:

  • An example is the sleep-wake cycle, it ensures that we are asleep and awake for one period each day.
  • The SCN is located within the hypothalamus and has responsibility for controlling our body clock. It is an endogenous pacemaker and is known as 'the master clock' as it controls all other body clocks.
  • It is self-regulating, ie the circadian rhythm is inbuilt into its cells, probably being the product of inherited genetics that have evolved to ensure our survival.
• • The SCN needs an environmental stimulus to keep it synchronised so it uses light to 'set' the body clock- light is the exogenous zeitgeber.
  • Light sensitive cells in the eye detect brightness & transmit this information to the SCN, which activates the pineal gland.
  • The pineal gland releases the hormone melatonin as brightness decreases, which promotes sleepiness- melatonin levels then decrease as light increases, to wake us up.
• Other Exogenous zeitgeber:

  • social cues- meal times & social activities.
  • Klein & Wegmann (1974) found that air travellers overcome jet lag more quickly if they spent more time outside at their destination- presumed that this is because they were exposed to more social cues which help synchronise the body clock.
• Circadian Rhythms Strength:

  • There's research support for the role of the SCN as an endogenous pacemaker- Morgan (1995) bred a strain of hamsters so that they had abnormal circadian rhythms of 20 hours rather than 24 hours.
  • SCN neurons from these abnormal hamsters were then transplanted into the brains of normal hamsters.
  • These normal hamsters then displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the recipients' brains.
  • Shows the importance of the SCN.
• Circadian Rhythms Weakness:

  • There's individual differences in circadian rhythms- research found the cycle length can vary from 13 to 65 hours (Czeisler et al)
  • Another indiviual difference: Cycle onset - individuals appear to be innately different in terms of when their circadian rhythms reach their peak. Duffy et al found that 'morning people' prefer to rise & go to bed early (about 6am- 10pm) whereas 'evening people' prefer to wake & go to bed later (10am- 1am).
  • This undermines the notion of an evolved biological internal clock, as it would be the same for everyone.
• Circadian Rhythms Strength:

  • There's research support for the role of the endogenous pacemakers in the sleep-wake cycle- Siffre (1975) was a cave expert who investigated the natural duration of his own sleep-wake cycle by conducting a study removed of exogenous zeitgebers.
  • He hid in a dark cave for 6 months (no light) with no clock/ reference to time, but did have verbal contact with outside world.
  • His sleep-wake cycle naturally settled at around 25 hours, but sometimes ranged up to 48 hours.
• Circadian Rhythms Strength Continued:

  • Siffre concluded that the absence of natural daylight allowed his endogenous pacemaker (SCN) to run at its natural rate, showing that it is the basic controller.
  • Daylight acted as an exogenous zeitgeber to stay synchronised to the day- night cycle.
  • The results indicate that our natural circadian rhythm of sleep-wake is biologically inbuilt.
• Circadian Rhythms Weakness:

  • There's methodological issues with Siffre's study- it was a case study & so cannot be generalised to other people, but has been repeated several times by other researchers.
  • There's a question of internal validity as Siffre was not isolated from all exogenous zeitgebers (had contact with the outside world & the effect on his sleep-wake cycle of even dim artificial light was not fully understood).
  • Issues mean we need to tread carefully when drawing conclusions from Siffre's work.
• Ultradian Rhythms:

  • An example is the stages of sleep- sleep consists of 5 distinct stages each with a different brain wave pattern.
  • Stage 1 & 2- lightest sleep
  • Stage 3 & 4- body is repairing & replenishing itself, growth hormones are released in stage 4, deepest sleep
  • Stage 5- vivid dreaming, mind thought to be repairing itself.
  • We cycle through the 5 stages a number of times whilst we sleep with each cycle lasting about 90 mins, average number of cycles per night is 4-6.
• Ultradian Rhythms Strength:

  • There's evidence that individuall differences in sleep are biologically determined, eg endogenous pacemaker.
  • Tucker et al (2000) studied PPs over 11 days, looking at sleep onset, duration & time in each stage.
  • Found that environmental factors such as room temperature or sleep hygiene had less impact than biology.
  • Led to conclusions that individual differences in sleep are biologically determined and maybe genetic in origin.
• Ultradian Rhythms Strength:

  • There's evidence for the important role of sleep indicating a potentially naturally selected biological origin.
  • Lack of sleep is known to affect concentration, cognitive performance & mood.
  • There is a growing body of evidence to show the relationship between sleep loss and cardiovascular disease (Ayas et al 2003).
  • This may indicate that the sleep cycles have evolved to ensure adequate human functioning.
• Infradian Rhythms:

  • Examples include: menstrual cycle, hibernation, migration, seasonal affective disorder.
• The Menstrual Cycle:

  • Lasts approximately 28 days & involves the fluctuation of levels of hormones released from the pituitary gland & ovaries.
  • These hormones control menstruation, ovulation & the preparation of the uterus for the possible implantation of an embryo.
  • Menstruation signals day 1 of the cycle
  • Ovulation typically occurs between days 11-14
  • This clear physiological control indicates the importance of endogenous pacemakers in controlling the rhythm & the relative independence from the influence of exogenous zeitgebers.
• The Menstrual Cycle:

  • FSH secreted by pituitary gland, causes an egg to mature in an ovary & stimulates ovaries to release hormone oestrogen.
  • LH secreted by pituitary gland, causes the mature egg to be released from the ovary.
  • Progesterone secreted by ovaries, maintains the lining of the uterus & stays high during pregnancy.
  • Oestrogen secreted by ovaries, stops FSH being produced & stimulates the pituitary gland to release hormone LH.
• Infradian Rhythms :/

  • There's evidence to suggest that exogenous cues can alter the menstrual cycle. When several women live together their menstrual cycles tend to synchronise- indicating external factors can influence the rhythm.
  • Russel et al (1980) found that if they took sweat from one woman and put it on the upper lips of another separate woman, then their menstrual cycles became synchronised.
  • Despite the obvious role of physiological control from hormone levels, there is evidence for the influence of exogenous zeitgebers too.
• Infradian Rhythms Strength:

  • The menstrual cycle impacts mating behaviours.
  • Miller et al (2007) found that lap dancers earn more tips during days that they are fertile compared to when menstruating.
  • Indicates the importance of changes during the cycle in human reproductive behaviour.
• Consequences of disruption:

  • Short term: lack of adequate sleep can affect judgement, mood, ability to learn & may increase the risk of serious accidents and injury.
  • Long term: chronic sleep deprivation may lead to a host of health problems including obesity, depression, diabetes cardiovascular disease, cancer & early mortality.
  • Can also be linked to absenteeism from work & lower productivity at work.