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 Google what core sleep means, you’ll get a definition that is entirely opposite from how Apple uses the term. So let’s break it down.
Let me give you a straightforward explanation of what you’re seeing when you look at your Apple sleep data.
Apple says its watch can tell the difference between four different states:
Light (“core”) sleep
REM sleep
In other words: Apple's definition of "core sleep" is identical to scientists' definition of "light sleep." It is otherwise known as N2 sleep. (More on that in a minute.)
The label Core was chosen to avoid possible unintended implications of the term light, because the N2 stage is predominant (often making up more than 50 percent of a night’s sleep), normal, and an important aspect of sleep physiology, containing sleep spindles and K-complexes.
A chart on the same page lays it out: non-REM stages 1 and 2 fall under the Apple category of “core” sleep, while stage 3 is “deep” sleep. That’s how Apple defined it in testing: If an EEG said a person was in stage 2 when the watch said they were in “core,” that was counted as a success for the algorithm.
What are the known sleep stages, and where does core sleep fit in?
N1 only lasts a few minutes. You’re breathing normally. Your body is beginning to relax, and your brain waves start to look different than they do when you’re awake. This would be considered part of your “light” sleep. The Apple Watch considers this to be part of your core sleep stage.
Non-REM stage 2 (N2)
N3 is often called “deep” sleep, and this stage accounts for about a quarter of your night. It has the slowest brain waves, so it’s sometimes called “slow wave sleep.” It’s hard to wake someone up from this stage, and if you succeed, they’ll be groggy for a little while afterward. This is the stage where the most body repair tends to happen, including muscle recovery, bone growth in children, and immune system strengthening. As we age, we spend less time in N3 and more time in N2.
REM sleep
REM sleep is so named because this is where we have Rapid Eye Movement. Your body is temporarily paralyzed, except for the eyes and your breathing muscles. This is the stage best known for dreaming (although dreams can occur in other stages as well).
Using Apple's definition, in which core sleep is the same as light sleep, it's normal for almost half of your sleep to be core sleep. Sleep scientists give an approximate breakdown (although the exact numbers may vary from person to person, and your needs aren't always the same every night):
N2 (light or "core" sleep): About 45%, so just under four hours if you normally sleep for eight hours
REM: About 25%, so also about two hours.
How to get more core sleep
That said, the best way to get more core sleep is to get more and better sleep in general. Start with this basic sleep hygiene checklist. Among the most important items:
Have a consistent wake-up time.
Keep your bedroom dark and cool.
Improving your sleep overall will improve all your sleep stages, whether your Apple Watch can tell them apart or not.
Other ways people use the term “core sleep”
In the 1980s, sleep scientist James Horne proposed that your first few sleep cycles (taking up maybe the first five hours of the night) constitute the “core” sleep we all need to function. The rest of the night is “optional” sleep, which ideally we’d still get every night, but it’s not a big deal to miss out from time to time. He described this idea in a 1988 book called Why We Sleep (no relation to the 2017 book by another author) but you can see his earlier paper on the topic here. He uses the terms “obligatory” and “facultative” sleep in that paper, and switched to the core/optional terminology later.
From there, somehow the internet has gotten the idea that N3 and REM are considered “core” sleep. I don’t know how that happened, and I don’t see it when I search the scientific literature. I have seen it on “what is core sleep?” junk articles on the websites of companies selling weighted blankets and melatonin gummies.
So, to wrap up: Core sleep, if you’re a napper, is the longest block of sleep you get during a day. Core sleep, to scientists who study sleep deprivation, is a hypothesis about which part of a night’s sleep is the most important. 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, or light sleep.
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