Biological Rhythms

Information about biological rhythms in Psychology, especially relating to sleep.

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  • Created by: Isya
  • Created on: 26-05-09 18:09

Outline of rhythms...

CIRCADIAN - 24hr rhythm (eg sleep/wake cycle)

ULTRADIAN - shorter than 24hr rhythm (eg stages of sleep)

INFRADIAN - longer than 24hr rhythm (eg hibernation)

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SIFFRE cave study

Michel Siffre - the cave study

Siffre spent several days in a cave with no exogenous zeitgebers, (external time-givers,) such as light or a watch, to tell him what time of day it was.

Researchers found that his sleep/wake pattern settled into a rhythm of slightly over 25 hours, instead of the usual 24 hours.

This suggests that our natural sleep/wake cycle is longer, but zeitgebers continually 'reset' our biological rhythms to fit in with the day/night cycle

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SIFFRE cave study


  • Aschoff and Weber performed a similar study with students in an underground bunker, with similar findings


  • Siffre's study only used one participant and so results cannot be generalised
  • There may be individual differences
  • A cave is not a natural human environment
  • Lack of social interaction may have altered behaviour
  • Lacks external validity

(GEEZ, a lot more bad things than good! ^w^; )

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So what is the SCN?

Mammals have a pineal gland in the brain which releases the hormone melatonin.

Studies show that melatonin induces sleep, for example Abraham (2000) who found that sparrows fell asleep when injected with the hormone. Melatonin production decreases, however, when it is light.

In birds, the pineal gland is near the skull. As the skull in this area is very thin, light can get through to the pineal gland directly and activate a decrease in melatonin production. In humans, and more evolved mammals, however, the pineal gland is in the centre of the brain. This means that no light can reach it directly.


This is when the SCN comes in! (Woo!)

The suprachiasmatic nucleus is, as discussed before, above the optic chiasma. Light reaches the eyes and this triggers reactions down the optic paths to the visual cortex, passing through the optic chiasma. The SCN picks this up and 'signals' to the pineal gland.


Hence why light can effect our sleep/wake cycles :)

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