Sleep is essentially a unitary, passive state produced by a general reduction of brain activity.

From the outside, sleep looks like inactivity. A sleeping person lies still, responds less to the environment, and later often remembers little of the interval. That made it easy to think of sleep as a uniform state in which the brain simply powered down.

Electroencephalography changed the picture. Recordings from sleeping brains showed organized electrical patterns that changed repeatedly through the night rather than one steady low-activity state.

The discovery of rapid eye movement sleep in 1953 made the contrast even sharper. During REM, the brain shows distinctive activity, vivid dreaming is common, the eyes move rapidly, and skeletal muscles are strongly inhibited. Non-REM sleep itself contains several stages with different physiological characteristics.

Sleep is now understood as an actively regulated cycle generated by interacting brain systems and circadian and homeostatic processes.

Some regions do become less active during sleep, and responsiveness to the outside world is reduced. The overturned claim is that those facts amount to a general neural shutdown.

Sleep is a different operating state, not the absence of brain operation.