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Psychology Revision Biological Rhythms, Sleep and Dreaming
A rhythm is something that is regularly repeated e.g. eating several times a day. These rhythms are
repeated over different intervals and they are governed by both internal (endogenous) and external
Endogenous Pacemaker = something internal that sets the rhythm
Exogenous zeitgeber = something external that affects / sets the rhythm e.g. light
Definition: A circadian rhythm is 24-hour cycle that the body has for example temperature, heart
rate and the sleep wake cycle.
The Suprachiasmitic Nucleus (SCN) is a small group of cells in the hypothalamus; it lies just above the optic
chasm so it can gain input fairly directly from the eyes.
Morgan removed the SCNs from hamsters and found that their circadian rhythms disappeared. He also
transplanted that SCNs from hamsters who had mutant circadian rhythms to `normal' hamsters. He found
that the hamster receiving the transplant took on the mutant circadian rhythms.
Silver et al also found that if SCNs are transplanted into an animal who's SCN has been removed; the
animal receiving the transplant have a restored circadian rhythm.
This is a rhythm that last less than 24 hours, an example is the sleep cycle. We all sleep in a cycle of 5
An infradian rhythm lasts for more than a day for example the menstrual cycle which occurs once a month.