Here’s What ‘Core Sleep’ Really Means, According to Your Apple Watch

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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.

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