Sleep is a vital, complex state with as yet unknown functions. It is active and highly organized and is regulated by homeostatic, circadian, and ultradian processes. It consists of two distinct sleep states, rapid eye movement (REM) and non-rapid eye movement (NREM), both of which have a dramatic impact on many aspects of physiology and behavior. The significance and consequences of the REM-NREM organization of sleep are not known. On the other hand, the sleep state and its organization are quite fragile and dynamic. Any number of factors can disrupt sleep and its expression, and its nature changes over the life span. What is certain is that any reduction and/or disruption of sleep hinders an organism's ability to navigate through the waking state.