Consciousness is a state of brain function. It can be described in two or more dimensions. The two principal dimensions being level (wakefulness) and content (awareness). Healthy people are awake and aware during the day and during sleep they pass reversibly through different phases of reduced wakefulness and awareness. Many clinical conditions are associated with disorders of consciousness including traumatic brain injury, stroke, anoxia, infection, intoxication, and inflammation. The three main clinical states are coma, minimally conscious state, and vegetative state. Clinical examination is important when managing disorders of consciousness. Standardized testing schemes are used to identify the reason for the critical condition and to evaluate to which extent consciousness is affected. Instrumental tools such as brain scans and measures of brain electrical activity play an increasing role when working with disorders of consciousness both for diagnostic and prognostic purposes. Sleep is characterized by well-defined physiological states that can be described by brain electrical activity combined with measures of muscle activity, eye movement, and heart activity among others. The presence and change between states are referred to as sleep architecture. While sleep architecture follows a general pattern in healthy subjects, that is not the case in disorders of consciousness. During coma, sleep architecture is absent, while elements of normal sleep architecture may appear in the minimal conscious state and the vegetative state.