Biological rhythms: Circadian rhythms
These refer to biological rhythms that are subject to 24 hour cycles which
regulate a number of body processes such as the sleep/wake cycle and
body temperature.
The sleep wake cycle
Feelings of drowsiness when its night, and alertness in the day demonstrates the effects of daylight
(exogenous zeitgeber). Sleep/wake cycle is governed by an internal endogenous (endogenous
pacemaker) a biological clock called the 'superchiasmatic nucleus' (SCN) , this co-ordinates the
activity of circadian rhythms.
Siffre's cave study
Aimed to see the effects on biological rhythms if it was 'left to its own devices'
without the influence of external stimuli (exogenous zeitgeber) such as light. Siffre
spent 2 months in a cave with no natural light and adequate food. He resurfaced in
september, yet thought it was august. Showing how his 'free running' biological
rhythm settled down to one that was just beyond 24 hours (25 hours), though was a
able to maintain a regular sleep/wake cycle, he did not perceive the circadian rhythm
as longer, leading to an inaccurate estimation of time.
Wever(1976)
Similar results conducted , convinced group of participants to spend 4 weeks in ww2
bunker deprived of natural light, all but one participant showed a circadian rhythm of 25
hours. Both studies suggest that the 'natural' sleep/wake cycle is slightly longer than 24
hours (without light), but a 24 cycle is entrained in us by exogenous zeitgebers associated
with a 24 hour day e.g. number of daylight hours, mealtimes etc.
Evaluation of Circadian rhythms
One strength of research into circadian rhythms is it provides understanding of the consequences
that may occur if said rhythms are desynchronised. E.g. night workers experience a drop in
concentration at 6am, meaning mistakes are more likely to occur. Further, late night shift work and
poor relationship seem to link for example, are 3x more likely to have heart disease than those with
typical work schedules. Therefore this research into sleep/wake cycle may have economic
implications in regards to employ productivity.
Counter-point: However, studies into the effects of shift work tend to use correlational methods
which assume causation, whereas this does not consider other factors. For example,
(Solomon)concluded that high divorce rates in shift workers may be due to the strain of deprived
sleep and lack of attendance to family events , suggesting it is not solely biological factors which
create adverse consequences in shift work employees.
Limitation: Studies of the sleep/wake cycle and free running biological rhythms usually include a
case study like that of Siffre, or small group samples like in Aschoff and Wever's bunker study. This is
These refer to biological rhythms that are subject to 24 hour cycles which
regulate a number of body processes such as the sleep/wake cycle and
body temperature.
The sleep wake cycle
Feelings of drowsiness when its night, and alertness in the day demonstrates the effects of daylight
(exogenous zeitgeber). Sleep/wake cycle is governed by an internal endogenous (endogenous
pacemaker) a biological clock called the 'superchiasmatic nucleus' (SCN) , this co-ordinates the
activity of circadian rhythms.
Siffre's cave study
Aimed to see the effects on biological rhythms if it was 'left to its own devices'
without the influence of external stimuli (exogenous zeitgeber) such as light. Siffre
spent 2 months in a cave with no natural light and adequate food. He resurfaced in
september, yet thought it was august. Showing how his 'free running' biological
rhythm settled down to one that was just beyond 24 hours (25 hours), though was a
able to maintain a regular sleep/wake cycle, he did not perceive the circadian rhythm
as longer, leading to an inaccurate estimation of time.
Wever(1976)
Similar results conducted , convinced group of participants to spend 4 weeks in ww2
bunker deprived of natural light, all but one participant showed a circadian rhythm of 25
hours. Both studies suggest that the 'natural' sleep/wake cycle is slightly longer than 24
hours (without light), but a 24 cycle is entrained in us by exogenous zeitgebers associated
with a 24 hour day e.g. number of daylight hours, mealtimes etc.
Evaluation of Circadian rhythms
One strength of research into circadian rhythms is it provides understanding of the consequences
that may occur if said rhythms are desynchronised. E.g. night workers experience a drop in
concentration at 6am, meaning mistakes are more likely to occur. Further, late night shift work and
poor relationship seem to link for example, are 3x more likely to have heart disease than those with
typical work schedules. Therefore this research into sleep/wake cycle may have economic
implications in regards to employ productivity.
Counter-point: However, studies into the effects of shift work tend to use correlational methods
which assume causation, whereas this does not consider other factors. For example,
(Solomon)concluded that high divorce rates in shift workers may be due to the strain of deprived
sleep and lack of attendance to family events , suggesting it is not solely biological factors which
create adverse consequences in shift work employees.
Limitation: Studies of the sleep/wake cycle and free running biological rhythms usually include a
case study like that of Siffre, or small group samples like in Aschoff and Wever's bunker study. This is
