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Biological rhythms can vary in length
Circadian rhythms ­ have one cycle that occurs
every 24 hours, e.g. the sleep-waking cycle.
Infradian rhythms ­ have cycles that are longer
than 24 hours, e.g. the menstrual cycle.
Ultradian rhythms ­ have cycles which are
shorter than 24 hours, e.g. the sleep cycle has
several repeating stages of light and deep
sleep.…read more

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Biological rhythms are determined by
internal and external influences
Endogenous pacemakers ­ are genetically
determined biological structures and
mechanisms within the body which act as our
internal body clock.
Exogenous zeitgebers ­ are external
influences, like light, which act as prompts to
trigger biological rhythms.…read more

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Endogenous pacemakers
The suprachiasmatic nucleus (SCN), part of the
hypothalamus, seems to act as an internal clock to keep
the body on an approximate 24 hour sleep-waking cycle.
It is sensitive to light and regulates the pineal gland, which
secretes melatonin (a hormone which induces sleep).
When there is less light, more melatonin is produced.
When there is more light, secretion is reduced and waking
occurs.
Menaker et al (1978) lesioned this structure in hamsters
and found that their sleep-waking cycle was disrupted.…read more

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Exogenous zeitgebers
These are the influences from outside the body
and act like a prompt, which may trigger a
biological rhythm.
Light is considered to be the most important
zeitgeber. Siffre (1975) spent six months in a
cave. He had no clocks and no natural light as
zeitgebers. His sleep-waking cycle extended
from a 24 hour cycle to a 25-30 hour cycle. It
therefore seems that natural light is needed to
fine-tune our normal 24 hour cycle.…read more

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Endogenous and exogenous factors interact
In some cases, endogenous factors may
completely determine a cycle. Pengelly and
Fisher (1957) found that squirrels will hibernate
even when kept in laboratory conditions very
different from their natural environment.
However, many animals can react more flexibly,
especially humans who are able to adapt to
their surroundings. We can make ourselves
stay awake and change the environment to suit
our needs.…read more

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