Describe the effect of endogenous pacemakers and exogenous zeitgebers on
the sleep/wake cycle. (16 marks)
Biological rhythms are influenced by two things endogenous pacemakers (internal body
clocks) and exogenous zeitgebers (external cues in the environment) which reset the sleep-
wake cycle every day to maintain coordination with the external world.
Endogenous pacemakers are the internal body clocks that regulate many of our biological
rhythms, the circadian the sleep/wake cycle. The suprachiasmatic nucleus (SCN) is a tiny
bundle of nerve cells located in the hypothalamus and is the main endogenous pacemaker
and it maintains the sleep/wake cycle. The SCN receives information about light from nerve/
optic fibres and this continues even when our eyes are closed, enabling the biological clock
to adjust to changing patterns of daylight while we are sleeping.
The SCN then sends the information to the pineal gland, which leads to an increase in the
production of melatonin at night, helping to induce sleep. The SCN and pineal gland
therefore work together as endogenous pacemakers, but their activity is responsive to the
external cue light.
Another important influence on biological rhythms is exogenous zeitgebers which are
external factors that reset our biological clocks. They can include social cue such as meal
times and social activities, but the most important zeitgeber is light as it can reset the body
main endogenous pacemaker the SCN and thus plays a role in the maintainace of the sleep/
wake cycle.
A strength of endogenous pacemakers is the importance of the SCN has been demonstrated
in research. For example, Morgan (1955) bred hamster so that they had circadian rhythms
of 20 hours rather than 24. He then transplanted these SCN neurons from these abnormal
hamsters into the brain of normal hamsters, which then displayed the abnormal circadian
rhythm of 20 hours, showing the transplanted SCN had imposed it pattern onto the
hamsters. Thus, this research demonstrates the significance of the SCN and how
endogenous pacemakers are important for biological circadian rhythms. However, a
limitation of this research is that it’s based on animals and as humans and animals are
biologically different, you are unable to generalise finding to humans as it may lack external
validity.
A strength of exogenous zeitgebers is there is research to supports it role. For example,
when Siffre (1975) returned from his underground stay in a cave with no clocks or lights, he
believed the date to be a month earlier than it was. This research suggests that his 24-hour
sleep-wake cycle was increased by the lack of external cues (exogenous zeitgebers), making
him believe one day was longer than it was. Thus, this highlights the impact of external
factors on bodily rhythms. However, a limitation of this case study was that it was an
individual case study, therefore it lacks generalisability to the wider population.
A limitation of exogenous zeitgebers is there influence may be overstated. For example,
Miles et al research on a blind man, with a circadian rhythm of 24.9 hours. Despite exposure
to social cues, his sleep wake cycle couldn’t be adjusted and he to take sedatives at night
the sleep/wake cycle. (16 marks)
Biological rhythms are influenced by two things endogenous pacemakers (internal body
clocks) and exogenous zeitgebers (external cues in the environment) which reset the sleep-
wake cycle every day to maintain coordination with the external world.
Endogenous pacemakers are the internal body clocks that regulate many of our biological
rhythms, the circadian the sleep/wake cycle. The suprachiasmatic nucleus (SCN) is a tiny
bundle of nerve cells located in the hypothalamus and is the main endogenous pacemaker
and it maintains the sleep/wake cycle. The SCN receives information about light from nerve/
optic fibres and this continues even when our eyes are closed, enabling the biological clock
to adjust to changing patterns of daylight while we are sleeping.
The SCN then sends the information to the pineal gland, which leads to an increase in the
production of melatonin at night, helping to induce sleep. The SCN and pineal gland
therefore work together as endogenous pacemakers, but their activity is responsive to the
external cue light.
Another important influence on biological rhythms is exogenous zeitgebers which are
external factors that reset our biological clocks. They can include social cue such as meal
times and social activities, but the most important zeitgeber is light as it can reset the body
main endogenous pacemaker the SCN and thus plays a role in the maintainace of the sleep/
wake cycle.
A strength of endogenous pacemakers is the importance of the SCN has been demonstrated
in research. For example, Morgan (1955) bred hamster so that they had circadian rhythms
of 20 hours rather than 24. He then transplanted these SCN neurons from these abnormal
hamsters into the brain of normal hamsters, which then displayed the abnormal circadian
rhythm of 20 hours, showing the transplanted SCN had imposed it pattern onto the
hamsters. Thus, this research demonstrates the significance of the SCN and how
endogenous pacemakers are important for biological circadian rhythms. However, a
limitation of this research is that it’s based on animals and as humans and animals are
biologically different, you are unable to generalise finding to humans as it may lack external
validity.
A strength of exogenous zeitgebers is there is research to supports it role. For example,
when Siffre (1975) returned from his underground stay in a cave with no clocks or lights, he
believed the date to be a month earlier than it was. This research suggests that his 24-hour
sleep-wake cycle was increased by the lack of external cues (exogenous zeitgebers), making
him believe one day was longer than it was. Thus, this highlights the impact of external
factors on bodily rhythms. However, a limitation of this case study was that it was an
individual case study, therefore it lacks generalisability to the wider population.
A limitation of exogenous zeitgebers is there influence may be overstated. For example,
Miles et al research on a blind man, with a circadian rhythm of 24.9 hours. Despite exposure
to social cues, his sleep wake cycle couldn’t be adjusted and he to take sedatives at night
