Biological Rhythms (Sleep)

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Biological Rhythms
Circadian Rhythms

A circadian rhythm is a biological rhythm that runs on a roughly 24 hour cycle.  One example is the sleep/wake cycle.  Humans sleep about 8 out of 24 hours.  Even under constant light conditions, animals keep a circadian rhythm (e.g. sleeping and eating) of around 24 hours.  This suggests that circadian rhythms are endogenous (internal) as they continue to run even when exogenous (external) zeitgebers are missing.  However, it seems that we rely on some exogenous zeitgebers (such as light) to entrain our circadian rhythms with a 24 hour day, otherwise they may become slightly out of sync.  The main endogenous pacemaker is the suprachiasmatic nucleus (SCN), a tiny cluster of neurons located in the hypothalamus in the brain.  It causes the pineal gland to secrete melatonin (a hormone that makes us sleepy) in response to low light conditions.  However, even in constant light conditions the free-running SCN still controls melatonin secretion to a roughly 24 hour cycle.

One strength of explanations of circadian rhythms is that they suggest a relationship between biology and the environment.  For example, they consider how the (biological) SCN works with the exogenous zeitgeber light to control the sleep/wake cycle.  As most psychologists agree that both of these factors usually play a part, a combination of the two is sensible.  These explanations can therefore be praised for taking a more holistic approach.

However, a weakness of explanations of circadian rhythms is that they are often nomothetic.  This means that they assume circadian rhythms are the same for everyone.  For example, Siffre’s research suggests a free-running sleep/wake cycle of 25 hours, but research since has shown some have a 24 hour cycle.  This is problematic because it appears a more idiographic approach would be more suitable, and by being nomothetic the explanations may be too simplistic.

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Evidence to support the idea of an endogenous pacemaker (the SCN) relying on an external zeitgebers (light) comes from the case study of Michel Siffre.  He spent 7 months in a cave under constant artificial light conditions.  His sleep/wake cycle began to free-run at about 25 hours.  This suggests that we have an exogenous pacemaker that controls the sleep/wake cycle, but we rely on exogenous zeitgebers to entrain it to the 24 hour world around us, thus supporting explanations of circadian rhythms.

One drawback of this research is that it is a case study.  This means that it ...

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