Light sleep

Light sleep, or NREM1 and NREM2, are sleep stages that serve little importance in keeping humans alive, although NREM2 may help develop muscle memory. Both also give the body a break from physical activity, and are similar to waking rest.

Unlike SWS, and in part REM, light sleep is quite pleasant to wake from.

Most people have an unavoidable light sleep buffer that makes at least 20-40% of their sleep light, no matter what.

NREM1: Transitional sleep from being awake

NREM1 is the light sleep stage seen when transitioning from wakefulness to sleep, most notably in the form of napawares. It can also happen during sleep when deeper sleep (NREM2 or vitals) can't happen due to an uncalm environment. During this stage, one is largely self-aware, but still most of the way to being fully asleep. It can also happen when deeper sleep is interrupted by a noise, or when the body thinks you should wake up now, letting you decide whether or not to actually do so.

NREM2: "Traditional" light sleep

NREM2 is also in part a transitional sleep stage, occurring between REM and SWS, but it is also a sleep stage that exists on its own, functioning as physical rest while not doing very much else. It is relatively easily sacrificed by the body to make more space for vitals, and light sleep debt does not exist as low light sleep schedules can be continued indefinitely. This is largely how sleep reduction works.