Scientists know all life has a circadian rhythm of about 24 hours, but no one knows why. An educated guess suggests the cycle is linked to the day length. However, the answer may not be this simple, as the earth has not always had a 24-hour day. 400 million years ago in the Cambrian, when animals evolved onto land, the day was about 21 hours long. The most popular theory today is that circadian rhythms are linked to changing oxygen levels in the air. Hours of sunlight affect this factor because plants only release oxygen in daylight. Scientists do not understand the exact reasons for this link.
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