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Biological Rhythms and Sleep

What is a biological rhythm?

          Cyclical changes in the way biological systems behave.

          Most obvious rhythm is the sleep-wake cycle – people and many animals go to sleep when it is dark and wake up when it is light.

          There are other many other rhythms, e.g. the opening and closing of flowers with daylight, seasonal patterns of activity in hibernating animals.

What controls these rhythms?

           Internal biological ‘clocks’, called endogenous pacemakers.

           External cues from the environment, called exogenous zeitgebers. These include sunlight, food, noise, or social interaction.

3 main types of rhythm

          Circadian rhythms – cycles lasting approx 24 hours e.g. Sleep/waking cycle, body temperature cycle, metabolic rate cycle 

          Ultradian rhythms – cycles shorter than 24 hours (or more than 1 cycle per 24 hours) e.g. Sleep cycles, hormone release, foraging

          Infradian rhythms – cycles lasting longer than 24 hours e.g. menstrual cycle

Circadian Rhythms

          These are rhythms lasting ‘about one day’.

          E.G. sleep-wake cycle, body temperature, urine production. Some


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