So let’s break it down. 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.

Sleep is a very important function for our body to restore and repair. We'll go through the five stages and what happens during each.

This stage makes up most of what Apple reports as your core sleep. Non-REM stage 3 (N3) N3 is often called “deep” sleep, and this stage accounts for about a quarter of your night.

You might have the impression you dream all night long — but that isn't necessarily the case. Find out when you're most likely to dream, based on your sleep cycle.

A new study finds that entering N2 sleep—a deep phase of non-rapid eye movement sleep—may help lead to more “eureka” ...

Does sleeping on an idea work? Here’s what science says. Scientists are finding experimental evidence that the transition between wakefulness and sleep is a portal for creative thought.

Healthy sleep cycles are essential in getting quality rest. I had my sleep monitored to find out exactly what these cycles look like.

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.