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Unconsciousness, more appropriately referred 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.

Loss of consciousness should not be confused with the notion of the psychoanalytic unconscious or cognitive processes (e.g., implicit cognition) that take place outside of awareness.

Loss of consciousness may occur as the result of traumatic brain injury, brain hypoxia (e.g., due to a brain infarction or cardiac arrest), severe poisoning with drugs that depress the activity of the central nervous system (e.g., alcohol and other hypnotic or sedative drugs), severe fatigue, and other causes.

Law and medicine

In jurisprudence, unconsciousness may entitle the criminal defendant to the defense of automatism, an excusing condition which allows a defendant to argue that they should not be held criminally liable for their actions or omissions. In most countries, courts are called upon to consider whether unconsciousness in a situation can be accepted as a defense; it can vary from case to case. Hence epileptic seizures, neurological dysfunctions and sleepwalking may be considered acceptable excusing conditions because the loss of control is not foreseeable, but falling asleep (especially while driving or during any other safety-critical activity), may not be because natural sleep rarely overcomes an ordinary person without warning.

In many countries, it is presumed that someone who is less than fully conscious cannot give consent to anything. This can be relevant in cases of sexual behavior, euthanasia or patients giving informed consent with regard to starting or stopping a treatment.

In certain countries, first responders, EMT, or paramedics must legally obtain consent from an injured person if (s)he is conscious during an emergency before they can assume patient care. In most situations where the injured person is deemed unconscious, consent is implied and the emergency service provider is free to assume patient care.

See also

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