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Unconsciousness is the condition of being not conscious—in a 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 a type of unconsciousness. Fainting due to a drop in blood pressure and a decrease of the oxygen supply to the brain is 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.
There is a theory that unconsciousness occurs when different regions of the brain inhibit one another.
Law and medicine 
In jurisprudence, unconsciousness may entitle the criminal defendant to the defense of automatism, i.e. a state uncontrollably of one's own actions, an excusing condition that allows a defendant to argue that they should not be held criminally liable for their actions or omissions . In most countries, courts must 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.
Laws in some countries require that first responders, EMT, or paramedics obtain consent from an injured person who is conscious before they initiate patient care. In most situations where the injured person is deemed unconscious, consent is implied and the emergency service provider is free to provide care.
See also 
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- Traumatic brain injury
- Consciousness after death
- Do Not Resuscitate
- Living will
- Shallow water blackout