You begin the night in non-REM sleep and spend most of your rest time there. It starts light, in the “N1” stage, and moves to the deep “N3” stage. During this progression, your brain becomes less responsive to the outside world, and it gets harder to wake up. Your thoughts and most body functions slow down. You spend about half a normal night’s sleep in the “N2” phase, when scientists think you file away long-term memories.