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See also Unconscious mind.

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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.

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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 what would otherwise have been actions or omissions which broke the law. Courts are called upon to consider whether "falling asleep" (especially while driving or during any other safety-critical activity) can be an acceptable defense because natural sleep rarely overcomes an ordinary person without warning; however incidents related to epileptic seizures, neurological dysfunctions and sleepwalking may be considered acceptable excusing conditions because the loss of control may not be foreseeable. For a detailed discussion, see automatism (case law).

On the other hand, someone who is less than fully conscious cannot give consent to anything. This is relevant in the case of sexual behavior (not allowed with such a person), and also in the case of a patient giving informed consent with regard to starting or stopping a treatment, and euthanasia.

In many countries, First Responders, EMT, or Paramedics must legally obtain consent from a conscious casualty during an emergency before they can assume patient care. If consent is denied, the EMS provider can legally do nothing. In the situation where the casualty is unconscious, in most cases, the EMS provider can assume patient care under the legal assumption of implied consent.

Some Christians believe that after death, the "soul" enters an unconscious state before resurrection at the Last Judgment, a belief known informally as soul sleep.

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• Fainting
• Coma
• Brownout or greyout, a partial loss of consciousness
• Traumatic brain injury
• Consciousness
• Sleep
• Hypnosis
• Do Not Resuscitate
• Living will
• Shallow water blackout
• Somnophilia