Biological Rhythms

3 Types of Rhythm:

  • Circadian (24 hours)
  • Ultradian (-24 hours)
  • Infradian (+24 hours)

Endogenous factors (Biological Clock) Exogenous zeitgebers (Light)

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Describe Two Biological Rhythms(9)

A Circadian Rhythm: The Sleep–Waking Cycle (every 24 hours)
The 24-hour sleep–waking cycle clearly illustrates that circadian rhythms depend on an interaction of physiological and psychological processes. Our fairly consistent sleep pattern suggests an internal or endogenous mechanism (the biological clock).

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The human menstrual cycle occurs every 28 days. It is controlled by the hormones oestrogen and progesterone (ovarian hormones). The hormones cause the release of the egg and thickening of the lining of the womb so that it is ready to receive a fertilised egg. If the egg is not fertilised the lining is shed and so menstruation is the outcome of a cycle of activity that prepares the body for conception.

Menstruation is an endogenous mechanism as it is controlled mainly by internal biological factors (the hormones) but exogenous factors (external cues) can also affect the rhythm.

An Ultradian Rhythm: The Stages and Cycles of Sleep

An ultradian rhythm repeats in a cycle of less than 24 hours. In sleep the cycles occur approximately every 90 minutes and the following stages occur within these cycles. Sleep is not a total loss of consciousness but there is a descent into reduced consciousness.

An Infradian Rhythm: The Menstrual Cycle (greater than 24 hours)
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