A conceptual model of sleep.

REASONS why sleep occurs and is necessary are not understood. Various models are more notable by their obvious inadequacy than by their reconciliation of a wide variety of observations. Both apparent common sense and the considered judgment of authorities "hardly doubt the metabolic basis of the nervous exhaustion . . . which requires for its elimination the recovery processes of sleep." 1 Yet this explanation has no solid base of observation to support it. One could postulate a general principle of epistemology: that if a given realm contains an important element which cannot be consistently related to the remainder of the realm, then the realm is poorly understood. Applying this principle to the realm of human and animal psychology, we could say the realm is poorly understood to the extent we are unable to relate sleep to the rest of the functions of the organism. To the extent