Biological rhythms
- Distinct patterns of changes in body activity that conform to cyclical time periods
- Influenced by internal body clocks (endogenous pacemakers) as well as external changes in
environment (exogenous zeitgebers)
- Some of these rhythms occur many times during a day (ultradian rhythms), others take longer than a
day to complete (infradian rhythms) and some take much longer (circannual rhythms)
Biological rhythms: Circadian rhythms
Circadian rhythms
- Biological rhythms, subject to/last for a 24-hour cycle, which regulate a number of body
processes such as sleep/wake cycle and changes in core temperature temperature
Examples of circadian rhythms:
1. Sleep/wake cycle
2. Core body temperature
The sleep/wake cycle
Evidence for the exogenous zeitgeber of light is that we feel drowsy at night time and alert during the
day
BUT sleep/wake cycle is also governed by an internal/endogenous pacemaker, which is a biological
‘clock’ called the suprachiasmatic nucleus (SCN)
- SCN lies just above the optic chiasm, which provides information from the eye about light, an
exogenous zeitgeber can reset the SCN
What if the biological clock was ‘left to its own devices’ without the influence of external stimuli such
as light? If we had no idea whether it was day or night, would we still fall asleep and wake up at
regular times?
Researchers have tried to answer this question…
Siffre’s cave study
Micheal Siffre is a self-style caveman who spent several extended periods underground to study the
effects on his own biological rhythms
In a cave, he deprived himself of exposure to natural light and sound, but had access to adequate
food and drink
Findings: Siffre resurfaced in mid-September (1962) after two months in the caves of the Southern
Alps believing it was mid-august!
A decade later, he spent 6 months in a Texan cave
In both cases, his ‘free-running’ biological rhythm settled down to one that was just beyond the
usual 24 hours (around 25 hours) though he did continue to fall asleep and wake up on a regular
schedule
++ More research
Aschoff and Wever (1976)
- Convinced a group of P’s to spend 4 weeks in a WW2 bunker, deprived of natural light
Findings: All but one of the P’s (whose sleep/wake cycle extended to 29 hours) displayed a circadian
rhythm between 24 and 25 hours
Both Siffre’s experience and the Aschoff’s and Wever’s study suggest that the ‘natural’ sleep/wake
cycle may be slightly longer than 24 hours, but it is ENTRAINED by exogenous zeitgebers associated
with our 24-hour day, such as daylight hours and typical meal times
, Despite this, we shouldn’t overestimate the influence of exogenous zeitgebers on our internal
biological clock
- Folkard et al (1985) studied a group of 12 people who agreed to live in a dark cave for 3
weeks, going to bed at 11.45pm and rising at 7.45am
- Over the course of the study, researchers gradually sped up the clock (unbeknown to the P’s)
so an apparent 24-hour day eventually lasted only 22 hours
- Only 1 P was comfortably able to adjust to the new regime
- Suggests the existence of a strong free-running circadian rhythm that cannot easily be
overridden by exogenous zeitgebers
Shift work
- Research into circadian rhythms provides an understanding of the adverse consequences
that occur when they are disrupted (desynchronisation)
- Boivin et al (1996) found night workers who engaged in shift work experienced a period of
reduced concentration around 6 am (a circadian trough) meaning mistakes, and accidents
were more likely
- Knutsson (2003) found a relationship between shift work and poor health. Shift workers are
3x more likely to develop heart disease than people who work more typical work patterns
- Shows that research into sleep/wake cycle may have real world economic implications in
terms of how to best manage worker productivity
Counterpoint
- However, studies investigating the effects of shift work tend to use correlational methods
- Difficult to establish whether desynchronisation of the sleep/wake cycle is actually a cause of
negative affects
- Solomon (1993) concluded that the divorce rate in shift workers might be due to strain of
deprive to sleep and other influences such as missing out on important family events
- Suggests that may not be biological factors that create the adverse consequences associated
with shift work
Medical treatment
- Research into circadian rhythms has been used to improve medical treatments
- Circadian rhythms, coordinate a number of the bodies basic processes (eg heart rate,
digestion and hormone levels)
- Please rise and fall during the course of the day, which has led to the field of
chronotherapeutics – how medical treatment can be administered in a way that corresponds
to a persons biological rhythms
- For example, aspirin as a treatment for heart attacks is most effective if taken last thing at
night
- Aspirin reduces blood platelet activity, and this can reduce the risk of a heart attack
- Bonten et al (2015) support this and say heart attacks are most likely to occur in the
morning, so the timing of taking aspirin matters
- Shows that research into the circadian rhythms and help increase effectiveness of drug
treatments
Individual differences
- Limitation of research into circadian rhythms is that generalisations are difficult to make
- For example, Siffre’s cave study was a case study so we cannot generalise the findings to all
people – lacks population validity
- It seems that sleep wake cycle is may vary widely from person to person
- Distinct patterns of changes in body activity that conform to cyclical time periods
- Influenced by internal body clocks (endogenous pacemakers) as well as external changes in
environment (exogenous zeitgebers)
- Some of these rhythms occur many times during a day (ultradian rhythms), others take longer than a
day to complete (infradian rhythms) and some take much longer (circannual rhythms)
Biological rhythms: Circadian rhythms
Circadian rhythms
- Biological rhythms, subject to/last for a 24-hour cycle, which regulate a number of body
processes such as sleep/wake cycle and changes in core temperature temperature
Examples of circadian rhythms:
1. Sleep/wake cycle
2. Core body temperature
The sleep/wake cycle
Evidence for the exogenous zeitgeber of light is that we feel drowsy at night time and alert during the
day
BUT sleep/wake cycle is also governed by an internal/endogenous pacemaker, which is a biological
‘clock’ called the suprachiasmatic nucleus (SCN)
- SCN lies just above the optic chiasm, which provides information from the eye about light, an
exogenous zeitgeber can reset the SCN
What if the biological clock was ‘left to its own devices’ without the influence of external stimuli such
as light? If we had no idea whether it was day or night, would we still fall asleep and wake up at
regular times?
Researchers have tried to answer this question…
Siffre’s cave study
Micheal Siffre is a self-style caveman who spent several extended periods underground to study the
effects on his own biological rhythms
In a cave, he deprived himself of exposure to natural light and sound, but had access to adequate
food and drink
Findings: Siffre resurfaced in mid-September (1962) after two months in the caves of the Southern
Alps believing it was mid-august!
A decade later, he spent 6 months in a Texan cave
In both cases, his ‘free-running’ biological rhythm settled down to one that was just beyond the
usual 24 hours (around 25 hours) though he did continue to fall asleep and wake up on a regular
schedule
++ More research
Aschoff and Wever (1976)
- Convinced a group of P’s to spend 4 weeks in a WW2 bunker, deprived of natural light
Findings: All but one of the P’s (whose sleep/wake cycle extended to 29 hours) displayed a circadian
rhythm between 24 and 25 hours
Both Siffre’s experience and the Aschoff’s and Wever’s study suggest that the ‘natural’ sleep/wake
cycle may be slightly longer than 24 hours, but it is ENTRAINED by exogenous zeitgebers associated
with our 24-hour day, such as daylight hours and typical meal times
, Despite this, we shouldn’t overestimate the influence of exogenous zeitgebers on our internal
biological clock
- Folkard et al (1985) studied a group of 12 people who agreed to live in a dark cave for 3
weeks, going to bed at 11.45pm and rising at 7.45am
- Over the course of the study, researchers gradually sped up the clock (unbeknown to the P’s)
so an apparent 24-hour day eventually lasted only 22 hours
- Only 1 P was comfortably able to adjust to the new regime
- Suggests the existence of a strong free-running circadian rhythm that cannot easily be
overridden by exogenous zeitgebers
Shift work
- Research into circadian rhythms provides an understanding of the adverse consequences
that occur when they are disrupted (desynchronisation)
- Boivin et al (1996) found night workers who engaged in shift work experienced a period of
reduced concentration around 6 am (a circadian trough) meaning mistakes, and accidents
were more likely
- Knutsson (2003) found a relationship between shift work and poor health. Shift workers are
3x more likely to develop heart disease than people who work more typical work patterns
- Shows that research into sleep/wake cycle may have real world economic implications in
terms of how to best manage worker productivity
Counterpoint
- However, studies investigating the effects of shift work tend to use correlational methods
- Difficult to establish whether desynchronisation of the sleep/wake cycle is actually a cause of
negative affects
- Solomon (1993) concluded that the divorce rate in shift workers might be due to strain of
deprive to sleep and other influences such as missing out on important family events
- Suggests that may not be biological factors that create the adverse consequences associated
with shift work
Medical treatment
- Research into circadian rhythms has been used to improve medical treatments
- Circadian rhythms, coordinate a number of the bodies basic processes (eg heart rate,
digestion and hormone levels)
- Please rise and fall during the course of the day, which has led to the field of
chronotherapeutics – how medical treatment can be administered in a way that corresponds
to a persons biological rhythms
- For example, aspirin as a treatment for heart attacks is most effective if taken last thing at
night
- Aspirin reduces blood platelet activity, and this can reduce the risk of a heart attack
- Bonten et al (2015) support this and say heart attacks are most likely to occur in the
morning, so the timing of taking aspirin matters
- Shows that research into the circadian rhythms and help increase effectiveness of drug
treatments
Individual differences
- Limitation of research into circadian rhythms is that generalisations are difficult to make
- For example, Siffre’s cave study was a case study so we cannot generalise the findings to all
people – lacks population validity
- It seems that sleep wake cycle is may vary widely from person to person
