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© 2011 by Anthony N. Nicholson. Our lives are ruled by time and we use time to tell us what to do. But the alarm clock that instructs us to wake or the wristwatch that tells us that it is time for lunch are unnatural clocks. Our bodies answer to another more persistent beat that probably started to tick shortly after life evolved. Embedded within the genes of humans, and almost all life on earth, are the instructions for a biological clock that marks the passage of approximately 24 hours. Biological clocks or ‘circadian clocks’ drive or alter our sleep patterns, alertness, mood, physical strength, blood pressure and every other aspect of our physiology and behaviour. Under normal conditions we experience a 24hour pattern of light and dark, and our circadian clock uses this signal to align biological time to the day and night. The clock is then used to anticipate the differing demands of the 24-hour day and ‘fine-tune’ physiology and behaviour in advance of the changing conditions. Body temperature drops, blood pressure decreases, cognitive performance falls and tiredness increases in anticipation of going to bed. Then, before dawn, metabolism is geared up in anticipation of increased activity when we wake.

Original publication





Book title

The Neurosciences and the Practice of Aviation Medicine

Publication Date



15 - 34