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Entrainment

The process by which a recurring environmental signal - usually a light cycle - resets an endogenous clock each day, holding it in step with the outside world. It is not optional, because a clock's natural period is almost never exactly a day: a human clock runs about twelve minutes slow daily, and morning light hauls back precisely that much. The essential point is that entrainment acts on the clock mechanism itself - unlike masking, where light merely switches behaviour on and off without ever touching the clock.