Circadian rhythms

• What is the circadian rhythm?

  • The main circadian rhythm is the sleep-wake cycle. Nearly all biological organisms have a biological 24 hour day.
  • It is governed by both external and internal signals.
• What does "endo" mean?

  "inside"
• What does "Exo" mean?

  "exit"
• What is the SCN categorised as?

  • The SCN is composed of many nerve cells within the hypothalamus.
  • The SCN acts as the master clock as it links to other brain regions that control sleep and arousal.
  • Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex.
  • The SCN lies just above the optic chasm and receives information about light. It continues even when our eyes are shut and the biological clock adapts to changing patterns of daylight while we sleep.
  • If our biological clock is running slow, the morning light automatically adjusts the clock. The circadian rhythm is put back in step with the world. 
• What is the pineal gland?

  • The SCN sends signals to the pineal gland, a small pea like structure behind the hypothalamus.
  • It is associated with production of melatonin, which is a sleep inducing chemical.
  • Melatonin levels decrease in the morning when it becomes light. Insomniacs treated with melatonin find it easier to get sleep.
  • It can also be used for jet lag
• Supporting evidence for the SCN from hamster study?

  • Supporting evidence for the SCN comes from Morgan, who removed the suprachiasmatic nucleus from hamsters, who lost their circadian rhythm suggesting that the SCN is an endogenous pacemaker.
  • However, it could be that by removing the SCN in surgery, it had damaged other parts in the hamsters’ brains.
  • Morgan then transplanted SCN cells from foetal hamsters into these ones, their circadian rhythms were then re-established again.
  • Morgan also transplanted SCN cells from the mutant hamsters (20 hour sleep cycle as compared to normal hamsters who had 24 hour sleep cycle) into normal hamsters.
  • The other hamsters took on the 20 hour sleep wake cycle of mutant hamsters.
  • However, the problem is that we may not be able to generalise the research as it is based on animals, human brains are much more complex so research supporting the SCN is flawed.
• Evaluation of the pineal gland from Schochat et al?

  • Supporting evidence for the pineal gland comes from a study by Schochat et al (1997).
  • 6 male p’s spent 29 hours between 7am and noon the next day in a sleep lab. They spent 7 minutes out of every 20 in complete darkness trying to sleep.
  • The sleep propensity (tendency to sleep) was measured at different times throughout. The greatest sleep propensity started in the late evening.
  • The levels of melatonin were measured by taking blood samples throughout.
  • There was a positive correlation between the urge to sleep and the level of melatonin in the participants.
  • A strength of this study was that it took place in a well controlled environment of the sleep lab.
  • However a correlation does not imply cause and effect so we cannot say that melatonin causes sleep propensity as sleep propensity may increase melatonin levels. Therefore, research supporting the pineal gland is flawed. 
• What are exogenous zeitgebers?

  These are environmental events that are responsible for resetting the biological clock - lights and social cues such as meal times
• What is light?

  • Light is the main exogenous zeitgeiber which can reset the internal body clock, receptors in the SCN are sensitive to changes in light.
  • Light plays a role in the sleep wake cycle but also in bodily processes like hormone secretion and blood circulation.
• What is a key study for showing the effect of light?

  • One study found that light may be detected by skin receptor sites on the body even when the same info is not received by the eyes.
  • 15 p’s were woken at various times and a light pad was shone on the back of their knees.
  • The researchers managed to produce a deviation in sleep/wake cycle by up to 3 hours, showing the strength of how power light is as an exogenous zeitgeber.
• What are social cues?

  • Meal times and social activities provide info about the time of day. Circadian rhythms begin at about the age of 6 weeks.
  • By about 16 weeks, the rhythm is set.
  • This is most likely due to the social cues set by parents such as prepared meals by adults and set bed times.
  • Jet lag can also be reduced by adapting to local times for eating and sleeping.
• Michel Siffre supporting study?

  • There is supporting evidence for the importance of exogenous zeitgebers from the Siffre study.
  • Michel Siffre spent long periods of time in a cave with no external clues to guide his circadian rhythm.
  • He woke, ate and slept when he felt it was appropriate to do so. It was found that without external cues of lights and clocks, Siffre’s sleep/wake cycle was settled down to 25 hour cycles, rather than the normal 24.
  • Furthermore, there was some dramatic variation, he resurfaced on 17th September 1962 but thought it was 20th August.
  • This suggests that light and other social cues are important in regulating our circadian rhythms and not just the internal body clock.
• Overstating of the effect of exogenous zeitgebers evaluation?

  • A problem is that the influence of exogenous zeitgebers may have been overstated.
  • Studies of people who live in the Arctic circle show normal sleep patterns dispute prolonged exposure to light.
  • This suggests there are occasions where exogenous zeitgebers will have little bearing on our circadian rhythms.
  • However, it could be argued that in these circumstances, other external cues such as social cues take over and influence the internal body clock instead of light .
• Strength of circadian rhythms regarding real life implications?

  • A strength of circadian rhythm research is that it has real life implications in the workplace.
  • For example, workers have reduced concentration in the morning as the strongest drive to sleep is between 2-4am, which is also when more mistakes and incidents are likely to occur.
  • In addition, shift workers are 3x more likely to develop heart disease which may be due to adjusting to different sleep wake patterns and a lack of good quality sleep in the daytime.
  • Therefore, there are economic implications in reducing illness and how best to manage worker productivity.
• Individual differences in research?

  • However, a problem with research into circadian rhythms is that there are individual differences.
  • It has been found that cycle length can vary from between 13-65 hours. Also some people are known for going to bed early and rising early (larks).
  • Some people are the opposite (owls). In addition, there are also age differences.
  • Teenagers and their circadian rhythms begin 2 hours after adults so current school times may mean that they have to wake up too early.
  • The research may not generalize to everyone. 
• What is the problem with most studies for circadian rhythms?

  • Most studies use small sample/ case studies. The sample amy not be representative so studies may lack population validity.
  • Even Siffre found that when he returned to the cave at 60 years old his internal body clock ticked more slowly than when he was a young man.
  • This is how even when the same person is involved, there are many factors affecting circadian rhythms so generalisation cannot be made.