Circadian rhythms

Periodic activity governed by: 

• Internal biological 'clocks' (endogenous pacemaker)
• External changes in the environment (exogenous zeitgeber) 

Circadian = lasts about 24 hours (e.g. sleep/wake cycle)

Exogenous zeitbegers = the fact that we feel drowsy when it's night-time and alert during the day shows the effect of daylight. Can reset the SCN. 

Endogenous pacemakers = 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 rhythm governed by the suprachiasmatic nucleus (SCN), which lies just above the optic chiasm and receives information about light directly from this structure. 

1962 = Spent 2 months in the caves of the Southern Alps. 

1970s = Spent 6 months 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 regualr sleep/wake cycle. 

A group of participants spent 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 zietgebers associated with our 24-hour day (e.g. number of daylight hours, typical mealtimes, etc.)

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 up when it said 7.45am. 

The researchers gradually sped up the clock so an apparent 24 hour day eventually only lasted 22 hours. 

Only one participant comfortably adjusted to the new regime. This suggests the existence of a strong free-running circadian rhythm that cannot be easily overridden by changes in the external environment. 

P - One strength of circadian rhythm research is practical application to shift work. 

E - Bovin et al (1996) found 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). 

CA - However, the individual differences between the participants' sleep/wake cycles mean that generalising the findings is an issue as cycles can vary, in some cases from 13 to 65 hours (Czeisler et al 1999). In addition, Duffy et al (2001) revealed that some people display a natural perference for going to bed early and rising early (larks) whereas others are the opposite (owls). There's also age-related differences. 

E - On the other hand, research into the sleep/wake cycle may have economic implications in terms of how best to manage worker productivity. 

P - One strength is the practical application to drug treatments. 

E - Circadian rhythms coordinate the body's basic processes (e.g. heart rate, hormone levels) with implications for pharmacokinetics (actions of drugs on the body and how well they are absorbed and distributed). Research shows that there are times during the day or night 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). 

CA - On the other hand, the poor control in research studies causes the validity of the findings to be questioned. Participants deprived of natural light still had access to artificial light (e.g. Siffre had a lamp he turned on when he woke up until he went to bed). Artifical light was assumed to have no effect on free-running rhythms whereas using it may be like taking a drug that resets particpants' biological clocks. This suggests that researchers may have ignored an important confounding variable. 

E - However, Czeisler et al (1999) adjusted participants' circadian rhythms from 22 to 28 hours using dim lighting, showing that the rhythm can still be altered without use of artificial light. Thus, research may have real-life medical benefits. 

P - One limitation is the use of case studies and small sample sizes. 

E - Studies of the sleep/wake cycle often use small groups of participants (e.g. Aschoff and Wever) or even single individuals (e.g. Siffre). Therefore, 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 aged 60n than when he was younger. 

CA - However, case studies provide a rich, detailed insight into the lives of the participants. The use of this idiographic method means that the results are catered to the aim of the study and can also be used to generate hypotheses for future study. 

E - Nevertheless, even when the same person is involved, there are factors which may prevent general conclusions from being drawn.