MODEL RELEASED. Abstract illustration representing the human sleep-wakefulness cycle; the pendulum swings between images of a girl asleep and awake. It has been suggested that feedback signals from the cerebral cortex and from the peripheral nerve receptors, by sustaining a particular state, might play an important role in controlling the sleep-wakefulness rhythm. Many biological processes are believed to take place under the direction of an internal clock mechanism. The regulation of biological time is poorly understood and is a subject of current (1990's) research.