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(2026 Exams) Biological Rhythms (A2 only)

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Biological Rhythms

The body has multiple natural rhythms that are controlled by a variety of factors, either internal or external. The rhythms can take place over different time scales - less than 24 hours or more than 24 hours.

Infradian rhythms

Infradian rhythms

  • These rhythms happen over a time period greater than 24 hours.
    • An example is the seasons, these occur over a year.
    • Another example is the menstrual cycle, which on average, takes place over 28 days.
Ultradian rhythms

Ultradian rhythms

  • These rhythms happen more than once over a 24 hour period.
    • Examples include pulse, heart rate and blood circulation.
Circadian rhythms

Circadian rhythms

  • These rhythms happen over a period of about 24 hours.
    • Our sleep-wake cycle, which is linked to our environment’s natural light-dark cycle, is perhaps the most obvious example of a circadian rhythm.
    • We also have daily fluctuations in heart rate, blood pressure, blood sugar, and body temperature.
    • Some circadian rhythms play a role in changes in our state of consciousness.

Control of Biological Rhythms

Control of rhythms is determined by external or internal factors: endogenous pacemakers are internal mechanisms that control natural rhythms; exogenous zeitgebers are external factors that control natural rhythms.

Sleep cycle - endogenous pacemaker

Sleep cycle - endogenous pacemaker

  • The endogenous pacemaker for sleep is the suprachiasmatic nucleus (SCN) which is a part of the hypothalamus.
  • The SCN signals for the pineal gland to produce the hormone melatonin which induces sleep.
Sleep cycle - exogenous zeitgeber

Sleep cycle - exogenous zeitgeber

  • The sleep-wake cycle is also controlled by exogenous zeitgebers. An example of an exogenous zeitgeber for sleep is light.
  • Light levels signal to the SCN to control levels of melatonin production.
    • If there is less light, more melatonin is produced.
    • If there is more light, less melatonin is produced.
  • The endogenous pacemaker and exogenous zeitgeber work together to control the sleep-wake cycle.
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