We may earn a commission from links on this page. Let's talk about one of the most confusing terms you’ll see on your fitness tracker—specifically your Apple Watch . Next to REM sleep, which you’ve probably heard of, and “deep” sleep, which feels self explanatory, there’s “core” sleep. And if you search or ask AI what core sleep means, you’ll often get a definition that is completely different from how Apple uses the term. 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. But in the scientific literature, "core sleep" is not a sleep stage at all. It can refer to a portion of the night that includes both deep and light sleep stages, and in other cases can be used to mean your longest or most important stretch of time in bed. There are a few other definitions, which I'll go into below.…