Sleep is a natural, recurring process that temporarily reduces the environmental and situational awareness of the individual. Although originally described as a passive activity, researchers and clinicians now characterize sleep as a very active and dynamic physiological and neurophysiological process. Sleep architecture is divided into non-rapid eye movement (NREM) and rapid eye movement (REM) stages. NREM sleep is divided into three separate stages (I, II, and III/IV). Stages I and II are characterized by drowsiness and the onset of light sleep, with slow but steady decreases in heart rate and body temperature. In contrast, stage III/IV consists of delta waves and is typically referred to as deep sleep. It is during this stage when it may be dicult to arouse the individual. If the person is awakened, he or she may be disoriented and confused for several minutes. Delta sleep is associated with the most frequent occurrences of bedwetting, sleepwalking, and night terrors.