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A biphasic system is one which has two phases.

For example, alcohol has a biphasic effect on the body, which is to say that its effects change over time. Initially, alcohol generally produces feelings of relaxation and cheerfulness, but further consumption can lead to blurred vision and coordination problems.

• In chronobiology, biphasic sleep is a form of polyphasic sleep in which sleep is broken up into two "naps," usually totalling less than the typical monophasic sleep system of almost all humans.