NREM sleep

Most of your night is not dreaming. For a healthy adult, roughly three quarters of total sleep is non rapid eye movement sleep, and the modern scoring system divides it into three stages that get progressively harder to wake from.

N1 is the doorway. It lasts a few minutes, brain activity slows a little, and if someone wakes you out of it you will often insist you were never asleep. The muscle twitches that jolt you back at the edge of sleep belong here too. N2 is where you spend the largest share of the night, commonly around half of total sleep, and it is defined by two signatures on an EEG: the sleep spindle, a short burst of fast activity, and the K complex, a single large wave. N3 is deep sleep, the slow wave stage, and it is the one that makes you hard to rouse.

How the three stages divide a night

The stages do not appear once and finish. They repeat, cycling with REM in a pattern that averages about 90 minutes but genuinely varies, both between people and from one cycle to the next in a single night. Early cycles are heavy on N3. Later ones drop most of it and hand the time to REM sleep instead, which is why the second half of the night has a different texture from the first.

That reshuffling is the point of the word sleep architecture. Two people can both sleep seven hours and have very different amounts of N2 and N3 inside them, and the same person can have a very different night after alcohol, a late flight or a week of short sleep.

Old scoring split this into four stages rather than three, with stages 3 and 4 later merged into N3, so an older book or an old sleep report may not match a current one. If you want the whole sequence walked through in order, start with sleep cycles explained. And when an app shows you a stage breakdown, check what produced it: on a phone only night Tock estimates stages from movement and sound rather than measuring them, and measured stages need Apple Health or a wearable.