Thanks to Coturnix for the barrage of insightful, educational posts.
From Clocktutorial #1, posted on A Blog Around the Clock:
But even within a single life stage, an organism needs to be adapted to more than one environment. A rabbit in a meadow experiences a very different environment during the day, during a dark night, and during a moonlit night. The same meadow is very different in winter from what it was last spring, summer or fall. A migratory bird's or whale's breeding grounds and overwintering grounds are likely to be very different from each other. A crab encounters a different beach during high and low tides. An organism has to have evolved biochemical, physiological and behavioral adaptations to all those disparate environments, as well as switches that turn these adaptations on and off at appropriate times, often very quickly. Because the switches have to act so fast, many of them have evolved to act independently of the environmental triggers. The environmental cycles, like day and night, tides, moon phases and seasons, are very predictable, thus a switch can get started in advance of the environmental change, thus rendering the organism "ready" for the new environment just in time for its appearance. Even if the organism is removed from the cyclical environment, the switches keep going on and off, and the physiological state of the organism keeps oscillating on its own, becoming a timer: a biological clock. The mechanisms of such oscillations, as well as various uses that organisms put their clocks to are studied by Chronobiology.