Amy Bishop
Discuss research into circadian rhythms. Refer to evidence in answer (16 marks)
Circadian rhythms are biological cycles lasting 24 hours after which a pattern of behavior
occurs/recurs. Examples of circadian rhythms include the sleep-wake cycle (alternating
states of sleeping and waking), and body temperature. The circadian rhythm of the sleep-
wake cycle is controlled by external environmental changes of light (exogenous zeitgebers)
that cause us to feel drowsy at night-time and alert during the daytime. And the body’s
internal biological clock (endogenous pacemakers) is free running and isn’t influenced by
external stimuli.
When light is detected in the morning: cells in the eye detect light so messages are sent via
the optic nerve to the suprachiasmatic nucleus (SCN) which sends messages to raise our
blood temperature and blood pressure and delay the release of hormones like melatonin
from the pineal gland. Also, early in the day, we have our highest level of cortisol which
makes us feel more awake. In the evening, as the sun disappears, the SCN picks up signals
of changing light from our eyes, so it sends messages to ensure the body temperature cools,
and sleep-inducing hormones like melatonin are released. So, in the evening our body
temperature begins to decrease and the secretion of melatonin increases, signaling that it is
time to sleep.
Siffre investigated the effects of the removal of light (exogenous zeitgeber) on the circadian
rhythm of the sleep and wake cycle. He spent 7 months in a cave without external cues of
light and sound to guide his rhythms so only his internal clock influenced his behavior. Every
day he took measurements of internal processes (like body temperature and pressure), and
simply ate and slept when he felt like it. His body eventually settled into a 25-hour circadian
rhythm despite getting rid of external cues. From this, we can conclude that the circadian
rhythm is free-running and will persist despite the removal of external cues. Indicating the
role of an endogenous pacemaker in the control of circadian rhythms.
Further research support for the role of the endogenous pacemaker in circadian rhythms
comes from Aschoff and Wever (1976). They got small groups of students to spend 4 weeks
in a WW2 bunker in the absence of exogenous zeitgebers, like light. The findings were that
most participants displayed a circadian rhythm between 24-25 hours, with some as long as
29 hours. This shows that the circadian rhythm operates even with the absence of external
cues (exogenous zeitgebers), and that the natural free-running cycle (which is only
controlled by the internal biological clock) may be a bit longer than 24 hours.
A strength of Siffre’s cave study is that it was highly controlled and both endogenous
pacemakers and exogenous zeitgebers are accounted for and controlled external cues, like
light and sounds. This experimental approach is important as it allows casual relationships to
be demonstrated. However, despite this studies into the circadian rhythm of the sleep/wake
cycle can also be criticised for low control as well. This is because, despite participants in
these studies being deprived of natural light, they still had access to artificial light, as it was
assumed to have no effect on free-running rhythms. For example, Siffre had a lamp turned
on from when he woke up to when he went to bed. This suggests that researchers may have
ignored an important confounding variable in circadian rhythm research.
Another strength of research into circadian rhythms is the practical application to shift work.
Boivin et al. found that shift workers experience a lapse of concentration around 6 am, so
mistakes and accidents are more likely to happen. This research is useful to employers as it
shows them when productivity is at its lowest and when incidents are more likely so they can
Discuss research into circadian rhythms. Refer to evidence in answer (16 marks)
Circadian rhythms are biological cycles lasting 24 hours after which a pattern of behavior
occurs/recurs. Examples of circadian rhythms include the sleep-wake cycle (alternating
states of sleeping and waking), and body temperature. The circadian rhythm of the sleep-
wake cycle is controlled by external environmental changes of light (exogenous zeitgebers)
that cause us to feel drowsy at night-time and alert during the daytime. And the body’s
internal biological clock (endogenous pacemakers) is free running and isn’t influenced by
external stimuli.
When light is detected in the morning: cells in the eye detect light so messages are sent via
the optic nerve to the suprachiasmatic nucleus (SCN) which sends messages to raise our
blood temperature and blood pressure and delay the release of hormones like melatonin
from the pineal gland. Also, early in the day, we have our highest level of cortisol which
makes us feel more awake. In the evening, as the sun disappears, the SCN picks up signals
of changing light from our eyes, so it sends messages to ensure the body temperature cools,
and sleep-inducing hormones like melatonin are released. So, in the evening our body
temperature begins to decrease and the secretion of melatonin increases, signaling that it is
time to sleep.
Siffre investigated the effects of the removal of light (exogenous zeitgeber) on the circadian
rhythm of the sleep and wake cycle. He spent 7 months in a cave without external cues of
light and sound to guide his rhythms so only his internal clock influenced his behavior. Every
day he took measurements of internal processes (like body temperature and pressure), and
simply ate and slept when he felt like it. His body eventually settled into a 25-hour circadian
rhythm despite getting rid of external cues. From this, we can conclude that the circadian
rhythm is free-running and will persist despite the removal of external cues. Indicating the
role of an endogenous pacemaker in the control of circadian rhythms.
Further research support for the role of the endogenous pacemaker in circadian rhythms
comes from Aschoff and Wever (1976). They got small groups of students to spend 4 weeks
in a WW2 bunker in the absence of exogenous zeitgebers, like light. The findings were that
most participants displayed a circadian rhythm between 24-25 hours, with some as long as
29 hours. This shows that the circadian rhythm operates even with the absence of external
cues (exogenous zeitgebers), and that the natural free-running cycle (which is only
controlled by the internal biological clock) may be a bit longer than 24 hours.
A strength of Siffre’s cave study is that it was highly controlled and both endogenous
pacemakers and exogenous zeitgebers are accounted for and controlled external cues, like
light and sounds. This experimental approach is important as it allows casual relationships to
be demonstrated. However, despite this studies into the circadian rhythm of the sleep/wake
cycle can also be criticised for low control as well. This is because, despite participants in
these studies being deprived of natural light, they still had access to artificial light, as it was
assumed to have no effect on free-running rhythms. For example, Siffre had a lamp turned
on from when he woke up to when he went to bed. This suggests that researchers may have
ignored an important confounding variable in circadian rhythm research.
Another strength of research into circadian rhythms is the practical application to shift work.
Boivin et al. found that shift workers experience a lapse of concentration around 6 am, so
mistakes and accidents are more likely to happen. This research is useful to employers as it
shows them when productivity is at its lowest and when incidents are more likely so they can
