Circadian rhythms

• 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). Others take more than a day to complete (infradian rhythms) and in some cases much longer (circannual rhythms).
• The circadian rhythm lasts for 24 hours, and the sleep/wake cycle is governed by internal and external mechanisms.

  Exogenous zeitgebers - the fact that we feel drowsy when it's night-time and alert during the day shows the effect of daylight.

  Endogenous pacemakers - a biological clock 'left to its own devices' without the influence of external stimuli called 'free-running'.
• There is a basic rhythm governed by the suprachiasmatic 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 cave study - French caver Siffre spend long periods in dark caves to examine the effect of free-running biological rhythms - two months in 1962 and six months in the 70s.

  In each case study, Siffre's free-running circadian rhythm settled down to just above the usual 24 hours.

  Importantly, he did have a regular sleep/wake cycle.
• Aschoff and Wever also found a similar circadian rhythm:

  A group of 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).
• One strength of circadian rhythm research is the practical application to shift work. Boivin found shift workers experience a lapse of concentration around 6 am (a circadian trough) so mistakes and accidents are more likely. Research also suggests a link between shift 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.
• One limitation is the use of case studies and small sample in studies. Studies of the sleep/wake cycle often use small groups of participants, or even single individuals. 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.
• Another limitation is the poor control in research 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 adjusted participants' circadian rhythms from 22 to 28 hours using dim lighting. Using artificial light may be like taking a drug that resent participants' biological clocks. This suggests that researchers may have ignored an important confounding variable in circadian rhythm research.
• A further limitation is that 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. Also, Duffy 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.