Unconsciousness, more appropriately referre to as loss of consciousness or lack of consciousness, is a dramatic alteration of mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is an illustration of unconsciousness. Fainting due to a drop in blood pressure and a decrease of the oxygen supply to the brain is an illustration of a temporary loss of consciousness. Loss of consciousness must not be confused with altered states of consciousness, such as delirium (when the person is confused and only partially responsive to the environment), normal sleep, hypnosis, and other altered states in which the person responds to stimuli.