Biological Rhythms

• Biological rhythms:

  • a biological rhythm is a pattern of change in the body that occurs in cycles and repeats
  • these biological rhythms often match daily, monthly or yearly cyclic changes in the environment
  • different biological rhythms have cycles that last different amounts of time for them to complete and then begin again
• Definitions of biological rhythms:

  • dian - per day
  • circa - around
  • infr - less frequent
  • ultra - more than
• Circadian rhythms:

  • happens once per day (repeats every 24 hours)
  • an example of this is the sleep wake cycle (happens once per day)
• Infradian rhythm:

  • happens less than once per day
  • an example of this is the menstrual cycle (happens every 28 days)
• Ultradian rhythms:

  • happens more than once per day
  • an example of this is the stages of sleep (happens multiple times a night)
• Ultradian rhythms:

  • an example of an ultradian rhythm is the stages of sleep
  • research has shown that we go through 5 stages of sleep per cycle
  • each cycle will last about 90 minutes in total
  • this cycle continues multiple times throughout the course of the night
  • the period of REM sleep (where dreaming occurs) gets longer in each cycle
• Stages of sleep:

  • stages 1 and 2 - this is light sleep when a person is easily woken. At the beginning of sleep brainwave patterns become slower and more rhythmic. These are called alpha waves. As sleep becomes deeper these become theta waves
  • stages 3 and 4 - this involves delta waves, which are slower still. This is deep sleep or slow wave sleep and is it difficult to wake someone at this point
  • stage 5 (REM sleep) - this is when the body is paralysed yet brain activity speeds up to similar levels as when we are awake. REM stands for rapid eye movement because at this stage eyes move around under the eyelids. REM is when dreaming occurs. Memories are formed in this stage
• Evaluation of ultradian rhythms:

  S - evidence to support - Dement and Kleitman

  S - practical applications - sleep disorders
• Research into ultradian rhythms:

  • Dement and Kleitman used EEGs to measure the sleep patterns of 9 participants
  • REM activity was highly correlated with dreaming and participants were able to recall their dreams during this period
• Evidence to support - Dement and Kleitman (identify):

  • there is research evidence to support ultradian rhythms and the different stages of sleep
• Evidence to support - Dement and Kleitman (explain):

  • Dement and Kleitman used EEGs to measure the sleep patterns of 9 participants
  • REM activity was highly correlated with dreaming and participants were able to recall their dreams during this period
• Evidence to support - Dement and Kleitman (conclusion):

  • This supports the existence of sleep stages as an ultradian rhythm, showing that there are clear stages of sleep which happen more than once per day, happening every 90 minutes during our sleep
• Practical applications - sleep disorders (identify):

  • research into the sleep cycle has led to the development of treatments for a range of sleep disorders
• Practical applications - sleep disorders (explain):

  • for example, understanding the different stages of sleep has led to helpful techniques and treatments for insomnia
  • this is helpful because it means we can use EEGs to identify their sleeping patterns and what stages of their sleep have been interrupted or when they are waking up
• Practical applications - sleep disorders (conclusion):

  • this shows that research into ultradian rhythms is helpful in the real world and has practical applications
  • insomnia diagnosis will be improved, leading to better therapies and potentially improving the economy if these people can get better sleep and become more productive at work
• Infradian rhythms:

  • an example of an infradian rhythm is the menstrual cycle
  • the menstrual cycle is governed by monthly changes in hormone levels and usually takes about 28 days to complete
  • a cycle begins on the first day of a woman's period (when the womb lining is shed) to the day before her next period
  • during each cycle, rising levels of the hormone oestrogen cause the ovary to develop an egg and release it (ovulation)
  • after ovulation, the hormone progesterone helps the womb lining grow thicker, preparing the body for pregnancy. If a woman is not pregnant, the womb lining sheds and leaves the body
• Evaluation of infradian rhythms:

  S - evidence to support - McClintock

  W - evidence to contradict - Yank and Schank
• Evidence to support - McClintock (identify):

  • there is evidence to support infradian rhythms and the menstrual cycle
• Evidence to support - McClintock (explain):

  • McClintock studied 29 women with irregular periods. Samples of pheromones were collected (armpits) from 9 of the women at different times in their cycle
  • these pheromones were then rubbed onto the lip of the other participants (the samples were treated so it was hygienic)
  • McClintock found that 68% of the women experienced changes to their cycle, which made them closer to the cycle of the donor who they got the pheromones from
• Evidence to support - McClintock (conclusion):

  • this supports the role of hormones in the control of the menstrual cycle, suggesting that biological rhythms are governed by internal processes
• Evidence to contradict - Yank and Schank (identify):

  • there is research evidence to contradict the influence of exogenous zeitgebers on the infradian rhythm, then menstrual cycle
• Evidence to contradict - Yank and Schank (explain):

  • A natural experiment by Yank and Schank studied 186 Chinese women living in university halls over a year
  • they found no evidence of women's cycles becoming synchronised
• Evidence to contradict - Yank and Schank (conclusion):

  • this is important because it suggests the Yank and Schank study shows that McClintock's findings are not fully representative of all women's menstrual cycles and that maybe factors other than pheromones control this biological rhythm
• Methodological extraneous variables (identify):

  • many of these studies investigating the effects of exogenous zeitgebers on menstrual cycles have methodological issues
• Methodological extraneous variables (explain):

  • there are many extraneous variables (sleep, diet, exercise) have been show to affect a woman's menstrual cycle
  • these are rarely controlled (like in McClintock's study)
  • this is a problem as there may be other factors that affect the timing of a woman's menstrual cycle
• Methodological extraneous variables (conclusion):

  • this is important as it's difficult to determine a cause and effect relationship on these studies on the menstrual cycle and infradian rhythms
  • more controlled studies need to be conducted (to control for diet/ stress)
• Circadian rhythms:

  • circadian rhythms last 24 hours
  • the sleep wake cycle is an example of a circadian rhythm
  • this refers to the patterns that you have every 24 hour period. So for most people this involves being awake during the day (light) and sleeping at night time (dark)
  • the sleep wake cycle is affected by both internal/biological factors and external/environmental factors
  • the internal factors are our endogenous pacemakers
  • the external factors are known as exogenous zeitgebers
• The effect of endogenous pacemakers on the sleep wake cycle:

  • this is the view that the sleep wake cycle is controlled by internal mechanisms

  this includes;

  • the suprachiasmatic nuclei (SCN)
  • the pineal gland and melatonin
• The suprachiasmatic nuclei (SCN):

  • one endogenous pacemaker is the suprachiasmatic nuclei (SCN). It is a small bundle of nerve cells found in the hypothalamus in each hemisphere of the brain
  • it is found just above the place where the optic nerves from each eye cross over. The SCN receives information about light from this area near the eyes
  • this means the SCN is affected by external cues (light and dark). However, it also has its own internal activity rhythm that persists even when isolated from the rest of the brain
• The pineal gland and melatonin:

  • the SCN passes information it receives about light to the pineal gland. The pineal gland is a pea like structure found just behind the hypothalamus
  • during the night the pineal gland converts the neurotransmitter serotonin into melatonin (melatonin is known as the 'Dracula hormone' as it only comes out at night)
  • when melatonin is released it acts on the brain's sleep mechanisms and makes you feel less alert and sleepy
• Research into circadian rhythms:

  • DeCoursey et al (2000) destroyed the SCN connections in the brain of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
  • the regular sleep wake cycle of the chipmunks disappeared and by the end of the study many had been killed by predators as they were awake and vulnerable to attack
  • this shows how important the SCN is in establishing and maintaining the circadian sleep wake cycle
• The effect of exogenous zeitgebers on the sleep wake cycle:

  • this is the view that the sleep wake cycle is controlled by external factors. When people are away from such external cues, they become desynchronised and have effects such as extreme tiredness, nausea, lack of concentration and reduced cognitive functioning
  • examples of exogenous zeitgebers include; light, caffeine, social cues, temperature changes, work schedules and exercise sessions
• Artificial light:

  • this stimulates our SCN and can dictate when we should be awake and asleep. It can reset the SCN (the body's main endogenous pacemakers) and so can disrupt the sleep wake cycle
• Social cues:

  • daily routines such as working hours (e.g 9-5) and mealtimes influence our sleep wake cycle
  • for example, time often dictates when we go to bed as well as just feelings of tiredness
  • therefore, social cues influence our circadian rhythms as well as our internal biology
• Interaction between endogenous pacemakers and exogenous zeitgebers on the sleep wake cycle:

  • the most commonly accepted view is that the endogenous pacemakers interact with the exogenous pacemakers (external cues) to ensure we are synchronised with the outside world
  • circadian rhythms such as the sleep wake cycle are capable of 'free running' which means that without any influence from external cues at all the SCN will run to a rhythm of about 25 hours. However, with the influence of the exogenous zeitgebers such as light and the existence of the 24 hour clock our sleep wake cycle is regulated at 24 hours
• Interaction between endogenous pacemakers and exogenous zeitgebers on the sleep wake cycle:

  • this process of the internal biological clock being adjusted by external cues is called entrainment. It is helpful to us as sometimes our external environment changes, for example if we fly a long way on holiday and cross time zones
  • therefore, our endogenous pacemakers provide us with a stable sleep wake cycle and exogenous zeitgebers provide us with a means for adjusting this cycle when the environment changes
• Evaluation of circadian rhythms and the effect on endogenous pacemakers and exogenous zeitgebers on the sleep wake cycle:

  S - evidence to support endogenous pacemakers - Siffre's cave

  evidence to support endogenous pacemakers - Ralph's hamsters

  S - practical applications - supporting shift workers

  S - evidence to support exogenous zeitgebers - Campbell and Murphy's knees
• Research to support endogenous pacemakers - Siffre's cave (identify):

  • there is research evidence to support the idea that endogenous pacemakers regulate the sleep wake cycle
• Research to support endogenous pacemakers - Siffre's cave (explain):

  • Michel Siffre was a cave explorer who spent 6 months in an underground cave to deprive himself of natural light and sound
  • this meant he was completely cut off from exogenous zeitgebers
  • after a period of time his sleep wake cycle settled to around 25 hours, not dissimilar from our standard 24 hour cycle in the real world
• Research to support endogenous pacemakers - Siffre's cave (conclusion):

  • this adds validity to the role of the endogenous pacemaker as this shows real world evidence for the role of the SCN in controlling the sleep wake cycle
  • even without environmental influences like light or social cues he was able to stick to a rough 24 hour sleep wake cycle