Deep Sleep State:

Sleep is a physical and mental state of rest during which we become inactive and unaware of our surroundings. We spend 75–80 percent of our sleeping time in non-REM (rapid eye movement) sleep.

Most physiological functions slow down. Body temperature, blood pressure, and breathing rate all drop. Sensory and motor activity become suspended, voluntary muscles become inactive, and we become totally or partially unconscious.

The brain, however, remains active throughout sleep—as active, according to some studies, as during waking.