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Biological rhythms
These are the cycles of activity that occur with
some regularity in an organism.
Circadian rhythms (24hrs): these cycles last 24 hours and repeat themselves. E.
g. This would be a sleep/wake cycle
Infradian rhythms (infrequent): these cycles occur less than once a day (last
more than 24 hours) e.g. Female menstrual cycle
Ultradian (more than 24hrs): these cycles occur more than once a day (and last
less than 24 hours) e.g. An example would be eating
Endogenous pacemakers: this is an internal `time giver' involving brain
mechanisms that control our biological cycles e.g. SCN
Exogenous zeitgebers: this is an external `time giver' involving external stimuli
that alter the biological clock on a daily basis. e.g. Light sources.…read more

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· Siffre (1972): spent 6 months in a cave with no time cues.
Artificial lights came on when he was awake. He settled
into a sleep-wake cycle of 25-30 hours. After 179 days he
thought 151 had passed. Shows endogenous pacemakers
have a large impact on circadian rhythms.
· Aschoff and Weber (1965) placed participants in a bunker
with no natural light. They settled into a sleep-wake cycle
of between 25 and 27 hours. This supports Siffre's study.
And also shows that endogenous pacemakers control the
sleep-wake cycle in the absence of light cues.
· Folkard et al (1985): isolated 12 participants from natural
light for 3 weeks manipulating the clock so that only 22
hours passed a day. Eleven participants kept pace with the
clock showing the strength of circadian rhythm as a free-
running cycle…read more

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· McClintock and Stern: found women who inhaled fumes from
armpit pads from other women who were about to ovulate had
shorter menstrual cycles while inhaling fumes from women who
had just ovulated made menstrual cycles longer. Pheromones in
the donors sweat affect the recipients infradian rhythm.
· Reinberg reported on a woman who spent 3 months in a cave
without natural lighting, . Her menstrual cycle shortened to 25.7
days, implying that infradian rhythms can be affected by
exogenous zeitgebers.
· Rosenzweig et al. Reported on Season Affective Disorder which
is when people feel low moods during winter darkness. It has
been associated with the darkness stimulating the production of
sleep, stressing the importance of light as and exogenous
zeitgeber.…read more

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· Klein and Armitage tested participants on verbal
and spatial tasks, finding that performance was
related to a 96 minute cycle.
· Gerkema and Dann found that ultradian rhythms
tend to be correlated with brain and body size,
with larger animals having longer cycles.
· Rechtschaffen and Kales measured electical
activity of the brain with an electroencephalogram
(EEG), finding different patterns of activity at
different times of sleep.…read more

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Main pacemaker is the superchiasmatic nucleus
(SCN), a small group of cells in the
hypothalamus that generates circadian
rhythm reset by light entering the eyes.
Morgan: found that removing SCN Ralph: took the SCN out of
from hamsters caused their genetically abnormal hamsters
circadian rhythm to disappear, but with a circadian cycle of only 20
when the SCN cells were hours, and put them in hamsters
transplanted in, the rhythm with the usual 24 hour cycle. The
returned. SCN therefore is a very normal hamsters cycle shortened
important endogenous to 20 hours suggesting endogenous
pacemaker. pacemakers play an important
role.…read more

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