circadian rhythms

• key term - biological rhythms

  distinct patterns of changes in the body activity that conform to cyclical time periods. Biological rhythms are influenced by internal body clocks (endogenous pacemakers) as well as external changes to the environment (exogenous zeitgebers)
• key term - circadian rhythm

  biological rhythms, subject to a 24hr cycle which regulate a number of body processes such as the sleep/wake cycle and changes in core temperature.
• biological rhythms

  All living animals are subject to biological rhythm and these exert an important influence on the way in which the body systems behave.

  All biological rhythms are governed by 2 things ->

  • the body's internal biological clock endogenous pacemakers
  • external changes in the environment known as exogenous zeitgebers.

  Some of these rhythms occur many times a day -> ultradian rhythms. Others take longer than a day to complete -> infradian rhythms and in some cases much longer -> circannual rhythms
• circadian rhythms

  Circadian rhythms last for around 24hrs. Circa = about diem = day. There are several important types of circadian rhythms e.g., sleep wake cycle and core body temperature
• the sleep/wake cycle

  The sleep/wake is governed by internal and external mechanisms.

  Exogenous zeitgebers – the fact that we feel drowsy when its night time and alert during the day shows the effect of daylight.

  Endogenous pacemaker – a biological clock ‘left to its own devices’ without the influence of external stimuli (e.g., light) is called ‘free running’.
• the sleep/wake cycle and the SCN

  
  

  The sleep wake cycle is also governed by an internal (endogenous) pacemaker - There is a basic rhythm governed by the suprachiasmatic nucleus (SCN), which lies above the optic chiasm and receives information about light directly from this structure.

  The exogenous zeitgeber (light) can reset the SCN.
• Siffre's cave studies

  Siffre demonstrated a free-running circadian rhythm of about 25 hours.

  French caver Siffre spent long periods in dark caves to examine the effects of free-running biological rhythms – 2 months (in 1962) in the caves of the Southern Alps and 6 months (in the 1970s) in a Texan cave (when he was 60).

  In each case study Siffre’s free running circadian rhythm settled down to just above the usual 24 hours (about 25 hours).

  Importantly, he did have a regular sleep/wake cycle.
• aschoff and weaver

  A similar circadian rhythm was found by Aschoff and Wever, who convinced a group of participants to spend 4 weeks in a WW2 bunker deprived of natural light. All but one (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24 and 25 hours.

  Siffre’s experience and the bunker study suggest that the ‘natural’ sleep wake cycle may be slightly longer than 24 hours but is entrained  by exogenous zeitgebers associated with out 24 hour day (e.g., number of daylight hours, typical mealtimes etc.)
• folkard 1985

  Endogenous pace makers may have stronger influence than exogenous zeitgebers.

  Folkard et al (1985) studied a group of 12 people who lived in a dark cave for 3 weeks, going to bed when the clock said 11.45pm and waking when it said 7.45am.

  The researchers gradually speeded up the clock (unbeknown to the participants) so an apparent 24 hour day eventually only lasted 22 hours.

  Only one participant comfortably adjusted to the new regimen. This suggests the existence of a strong free-running circadian rhythm that cannot easily be overridden by changes in the external environment.
• one strength of circadian rhythm research is practical application to shift work

  Boivin et al 1996 found that shift workers experience a lapse of concentration around 6am (a circadian trough) so mistakes and accidents are more likely. Research also suggests a link between shift work and poor health, with shift workers 3 times more likely to develop heart disease (Knutsson 2003).
• another strength is practical application to drug treatments

  Circadian rhythms coordinate the body’s basic processes (e.g., heart rate, hormone levels,) with implications for pharmacokinetics (action of drugs on the body and how well they are absorbed or distributed). Research shows there are times during the day when drugs are more effective. Guidelines have been developed for the timing of dosing for a range of drugs including treatments for cancer and epilepsy (Baraldo 2008). Thus research into circadian rhythms may have real-life medical benefits.
• one limitation is the use of case studies and small samples in the studies

  Studies of the sleep/wake cycle often use small groups of participant's e.g., Aschoff and Wever or even single individuals such as Siffre. Participants may not be representative of the wider population and this limits making meaningful generalisations. Siffre observed that his internal clock ticked much more slowly at 60 than when he was younger. This suggests that, even when the same person is involved, there are factors that may prevent general conclusions being drawn.
• limitation = poor control in research studies

  Participants deprived of natural light still had access to artificial light (Siffre had a lamp turned on from when he woke up to when he went to bed). Artificial light was assumed to have no effect on free running rhythms. But Czeisler et al 1999 adjusted participants circadian rhythms from 22 to 28 hours using dim lighting. Using artificial light may be like taking a drug that resets participants biological clocks. This suggests that researchers may have ignored an important confounding variable in circadian rhythm research.
• limitation = individual differences may be an influence on results 


  An issue complicating the generalisation of findings from studies of the sleep/wake cycle is that individual cycles can vary from 13 to 65 hours (Cziesler et al 1999). Also Duffy et al 2001, found some people display a natural preference for sleeping early (larks) but others prefer the opposite (owls). There are also age differences in sleep/wake patterns. This means that findings from sleep/wake cycle studies may not fully represent individual differences within the population