The first 3 stages, part of the first phase, are called non-REM sleep (N1 through N3). The final stage and second phase is REM. NREM stage N1. Stage N1 is the transition from wakefulness to sleep.

A chart on the same ... N3 (deep sleep): ... But if you’re just here because you were wondering what Apple Health or your Apple Watch's sleep app means by "core sleep," it means stages N1-N2, ...

Stage N1 sleep typically accounts for 5 to 10 percent or less of the total sleep time ... Stage N3 typically accounts for 10 to 20 percent of the total sleep time in young to middle-age adults and ...

On an Apple Watch, "core sleep" is another name for light sleep, which scientists also call stages N1-N2. It is not a type of deep sleep, and has no relation to REM.

NREM 2 sleep is deeper than N1, and we spend a lot more time (up to 50% of our sleep time) in this stage. Our brain activity changes during this time and we see sleep spindles and K-complexes.

N3, or slow-wave sleep, is the deepest stage of sleep. You spend 25% of your time asleep in this stage. As you age, though, you spend less sleep time in stage N3 and more time in N2.

The three sleep stages are N1 (light sleep), N2 (medium sleep), and N3 (deep sleep). “The human body cycles through four to five stages of sleep every 90 minutes overnight,” Dr. Vyas says.