Circadian Rhythms

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Circadian Rhythms

What does circadian mean?

—  This term comes from the Latin “circa”, meaning “about” and “dies”, meaning “day”.

—  There are many biological rhythms that vary regularly but following this basic 24 hour pattern.

—  These include; heart rate, urine secretion, metabolic rate, respiration (breathing) and temperature.

—  All the above cycles reach their peak at around 4p.m, and are at their lowest around 4 a.m.

—  Other cycles include hormone production and milk production in females.

The sleep-wake cycle

—  This is the most obvious circadian rhythm, as we normally follow a regular 24-hour cycle of sleeping and waking.

—  This regular pattern is maintained by exogenous (external) factors, cues such as light and dark.

—  These factors are known as zeitgebers (which is German for “time-givers”).

—  The role of zeitgebers was established in a classic study by the French explorer Michel Siffre...

Siffre (1972)

—  Aim: To establish the role of light as a Zeitgeber in the sleep-wake cycle.

—  Procedure: Siffre spent 7 months underground. He had adequate food and opportunities to take exercise and was always able to contact others by phone, but had no cues as to whether to it was


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