Here’s another explanation, which shows 4 stages of non-REM sleep!
Stage 1: you are in a “twilight” of drifting from awake to asleep.
Stage 2: Light sleep.
Stage 3: Almost slow-wave or delta wave sleep.
Stage 4: Extremely deep slow wave delta wave sleep. Sleep As Android or ANY sleep app has absolutely no way of telling the difference between stages three and four. The others are just guessing about stage 4. Sleep As Android includes both stages three and 4 as deep sleep, while counting stage 2 as “light” sleep. You ARE asleep, but easily awakened.
There is no way to measure the deepest stage 4 level of sleep with a sleep app that can’t access EEG readings. It’s just guessing on your Fitbit. So SaA combines stages 3 and 4 of non-REM sleep. That’s why you see so much of it. Stage 2 is “light.” And only stage 2.
