Circadian rhythms

• circadian rhythms are biological rhythms governed by endogenous pacemakers and exogenous zeitgebers
• biological rhythms are periodic activity, governed by:

  1. internal biological 'clocks' (endogenous pacemakers)
  2. external changes in the environment (exogenous zeitgebers)
• some of these rhythms occur many times a day (ultradian rhythms). other take more than a day to complete (infradian rhythms) and in some cases much longer (circannual rhythms)
• the circadian rhythms lasts about 24 hours
• there are several important types of circadian rhythms such as the sleep/wake cycle
• the sleep wake cycle is governed by internal and external mechanisms
• exogenous zeitgebers role in sleep/wake cycle:

  the fact we feel drowsy when its nighttime and alert during the day shows the effect of daylight
• role of endogenous pacemakers in sleep/wake cycle:

  a biological clock 'left to its own devices' without the influence of external stimuli (e.g. light) is called free running
• there is a basic rhythms governed by the suprachiasmaic nucleus (SCN) which lies just above the optic chiasm and receives information about light directly from this structure.

  the exogenous Zeitgeber (light) can reset the SCN
• Siffre's study:

  French caver suffer spent long periods in dark caves to examine the effects of free-running biological rhythms- two months in the caves of the southern alps and six months in a texan cave (when he was sixty)

  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 wever's study:

  a group participants spent four 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 our 24-hour day (e.g. number of daylight hours, typical mealtimes etc.)
• What study suggests endogenous pacemakers may have stronger influences than exogenous?

  folkard et al.
• folkard et al's study:

  studied a group of 12 people who lived in a dark cave for three weeks, going to bed when the clock said 11:45pm and waking when it said 7:45

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

  only one participant comfortably adjusted to the new regime. this suggests the existence of a strong free-running circadian rhythm that cannot easily be overridden by changed in the external environment
• strength of circadian rhythm research: practical application

  bolvin et al. found shift workers experience a lapse of concentration around 6am (a circadian trough) so mistakes and accidents are more likely. research also suggest a link between shifts work and poor health, with shift workers three times more likely to develop heart disease. thus, research into the sleep/wake cycle may have economic implications in terms of how best to manage worker productivity
• limitation: use of case studies and small samples

  studies of the sleep/wake cycle often use small groups of participants (e.g. aschoff and wever) or even single individuals (e.g. 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 studies

  participants deprived of natural light still had access to artificial light (e.g. siffre had a lamp turned on from when he woke up until he went to bed). artificial light was assumed to have no effect on free-running rhythms. but czeisler et al. 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 researcg
• limitation: individual differences

  an issue complicating the generalisation of findings from studies of sleep/wake cycle is that individual cycles can vary from 13 to 65 hours. also, Duffy et al. found some people display a natural preference for sleeping and rising early but others prefer the opposite. 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