Circadian Rythms

• Bio rhythms are distinct patterns of changes in body activity that conform to cyclical time periods. Rhythms are influence by internal body clocks (endogenous pacemakers) as well as external changes to  environment (exogenous zeitgebers)
• Circadian ryhtm is subject to a 24 hour cycle.
• Exams of circadian rhythms


  Sleep-wake cycle

  Metabolism

  Temperature

  Blood pressure
• IQ - Core body temperature varies by around 2 degrees centigrade during the course of a day. It's at its lowest around 4 in the morning (36°C) and peaks around 6 in the evening at 38°C. Evidence suggests that body temperature may have an effect on our mental abilities - the warmer we are (internally), the better our cognitive performance.
• IQ - Simon Folkard et al. (1977) demonstrated how children who had stories read to them at 3 pm showed superior recall and comprehension after a week compared to children who heard the same stories at 9 am. Similarly, Sunita Gupta (1991) found improved performance on IQ tests when participants were assessed at 7 pm as opposed to 2 pm and 9 am.
• All living organisms (plants, animals and people) are subject to bio rhythms which exert an influence on the behaviour of body systems. They are governed by:

  1. Endogenous pacemakers (The main pacemaker in mammals is the Suprachiasmatic Nucleus (SCN), a cluster of nerve cells located in the hypothalamus within limbic system of optic chiasm)

  
  

  2. Exogenous Zeitgebers (Light, temperature, social cues)
• Sleep-wake cycle - We tend to feel drowsy at night and alert during the day. This shows the effect of daylight and its role as an exogenous zeitgeber on the sleep/wake cycle.
• Siffre was a speleologist (cave expert) who investigated the natural duration of his own sleep- wake cycle. He Isolated himself from all daylight by hiding in a dark cave with only weak artificial light for 6 months (he had already done this for two months 10 years earlier where his 'free-turning' bio rhythm settled done to one that was just beyond 24 hrs. Although he did continue to wake and sleep on a regular schedule.  
• Siffre had no clock or any reference to time, however he did have verbal contact with the outside world. He had access to food and drink.
• Siffre findings - His sleep-wake cycle settled naturally at around 25 hours, although sometimes it would range up to 48 hours. Siffre concluded that the absence of natural daylight allowed his blological clock to run at its natural rate, and that normally daylight acted as an exogenous zeitgeber to resynchronise the cycle.
• Evaluation of Siffre - case study and cannot therefore be readily generalized to other ppl, however it has been repeated several times by other researchers. Also there is a question of internal validity as Siffre was not isolated from all exogenous zeitgebers as he had contact with the outside world, and the effect on his sleep-wake cycle of even dim artificial light was not fully understood.
• internal body clock - Folkard et al (1985) studied 12 people who lived in a dark cave for three weeks. They went to bed when the clock showed 11.45 pm and rose when it showed 7.45am. Over the 3 weeks the researchers gradually sped up the clock so it was running at 22 hours. None of the P's comfortably adjusted to new regime. Suggests a strong free running circadian rhythm is not as easily overridden by environmental influences.
• Evaluation - Knowledge of (disrupted) circadian rhythms (Desynchronisation) has practical application to shift work. The concentration of night workers is reduced at about 6am (a circadian trough) so mistakes and accidents are more likely (Boivin et al 1996), Shift workers are also 3x more likely develop heart disease (Knutsson 2003) perhaps because of the stress of readjusting to differen patterns of the sleep/wake cycle, and poor quality sleep during the day. This research has economic implications in terms of how best to manage worker productivity.
• Evaluation - However studies investigating effects of shift work tend to use correlation al methods so difficult to establish whether desynchronisation of sleep wake cycle is actually a case of neg effects.  E.g. divorce
• S - Circadian rhythms coordinate many basic body processes such as heart rate, digestion and hormones. These then have an effect on pharmacokinetics (the absorption and distribution of drugs in the body). There are certain peak times when drugs are likely to be more effective which has led to guidelines as to timings and dosage of drugs for cancer epilepsy, cardiovascular and ulcer treatments.
• W - The studies carried out involve small samples who may not be representative of the wider population. This limits meaningful generalisations which can be made. At the age of 60, Siffre himself, carried out a cave study again, and found his internal clock was slower than when he was younger. So there were different findings even when looking at the same person, which shows how hard it is to draw general conclusions.
• W - P's were deprived of natural light but did have artificial light, under the assumption that it would not have the effect of natural light. However, Czelsler et al (1999) adjusted P's CR's from 22 hours to 28 hours using dim lighting. Therefore the use of the light may have reset their biological clock meaning the study lacked validity as it was not measuring the true absence of exogenous zeitgebers.