Unconsciousness is the state of being completely unaware of your surroundings, unable to think, feel, or respond to what's happening around you. When someone is unconscious, they can't see, hear, or sense anything in a normal way, like a computer that's been switched off.

This happens when the brain temporarily stops its normal alert functioning. Someone might become unconscious if they hit their head hard, have a serious medical problem, or undergo surgery (where doctors use special medications to make patients unconscious on purpose so they won't feel pain). Sleep might seem similar, but it's different: sleeping people can wake up if you shake them or make noise, while unconscious people usually cannot.

Doctors and nurses treat unconsciousness as a medical emergency because they need to figure out why the person's brain stopped functioning normally and make sure the person is safe. The person usually cannot remember anything from the time they were unconscious, creating a complete blank in their memory.