Biological clock

The internal biological clock may determine the timing of an individual's behavior patterns. The clock functions independently of the environment, but is synchronized by external stimuli to changes in the external world. The biological clock affecting times of sleeping and wakefulness is synchronized to daytime wakefulness and nighttime sleep. Clocks representing the same day over six weeks show little variation in activity. In constant darkness the clock becomes "free running." It functions on a length of day greater than the normal twenty-four hours and results in the sleep period becoming later and later each day.