Friends, 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.
The root of the confusion lies in the fact that the term “core sleep” has been used in the scientific literature to mean a few different things. Importantly, it’s not a recognized sleep stage. Apple, on the other hand, decided to rename the sleep stages its watch can detect, and called one of them “core sleep”—but it bears no relation to any of the previous common uses of the term.
“Core sleep” in the Apple Watch is the same as light sleep
Let me give you a straightforward explanation of what you’re seeing when you look at your Apple sleep data.
Your Apple Watch tries to guess, mainly through your movements, when you’re in each stage of sleep. (To truly know your sleep stages would require a sleep study with more sophisticated equipment, like an electroencephalogram. The watch is just doing its best with the data it has.)
Apple says their watch can tell the difference between four different states:
- Awake
- Light (“core”) sleep
- Deep sleep
- REM sleep
These categories roughly correspond to the sleep stages that neuroscientists can observe with polysomnography, which involves hooking you up to an electroencephalogram, or EEG. (That’s the thing where they attach wires to your head). Scientists recognize three stages of non-REM sleep, with the third being described as deep sleep. That means stages 1 and 2, which are sometimes called “light” sleep, are being labeled as “core” sleep by your wearable.
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.)
So why didn’t Apple use the same wording as everyone else? The company says in a document on their sleep stage algorithm that they were worried people would misunderstand the term “light sleep” if they called it that.
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.
In other words, they thought we might assume that “light” sleep is less important than “deep” sleep, so they chose a new, important-sounding name to use in place of “light.”
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.
