sleep can follow a physiological or behavioral definition. in the physiological sense, sleep is a state characterized by reversible unconsciousness, special brainwave patterns, sporadic eye movement, loss of muscle tone (possibly with some exceptions; see below regarding the sleep of birds and of aquatic mammals), and a compensatory increase following deprivation of the state. in the behavioral sense, sleep is characterized by minimal movement, non-responsiveness to external stimuli (i.e. increased sensory threshold), the adoption of a typical posture, and the occupation of a sheltered site, all of which is usually repeated on a 24-hour basis. the physiological definition applies well to birds and mammals, but in other animals (whose brain is not as complex), the behavioral definition is more often used. in very simple animals, behavioral definitions of sleep are the only ones possible, and even then the behavioral repertoire of the animal may not be extensive enough to allow distinction between sleep and wakefulness.