It’s Time to Find Out What “core Sleep” Really Is.
Friends, let’s talk about one of the most confusing terms you’ll come across on your fitness tracker, the Apple Watch. Next to REM sleep, which you’ve probably heard of, and “deep” sleep, which is self-explanatory, there is “core” sleep. And if you Google what core sleep means, you’ll get a definition that’s completely opposite to how Apple uses the term. So let’s find out.
The root of the confusion is that the term “core sleep” has been used in the scientific literature to mean several different things. It’s important to note that this is not a recognized stage of sleep. Apple, on the other hand, has decided to rename the stages of sleep that its watch can detect, calling one of them “core sleep,” but it has nothing to do with any of the previous common uses of the term.
Apple calls light sleep “core sleep”
Let me give you a simple explanation of what you see when you look at Apple sleep data.
Your Apple Watch tries to guess, mostly based on your movements, when you’re in each stage of sleep. (To truly know your sleep stages would require a sleep study using more sophisticated equipment. The watch simply does the best it can with the data it has.)
Apple says its watch can differentiate between four different states:
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Awake
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Light (“main”) sleep
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Deep dream
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REM sleep
These categories roughly correspond to the stages of sleep that neuroscientists can observe using polysomnography, which involves hooking you up to an electroencephalogram, or EEG. (This is where the wires are attached to the head). Scientists distinguish three stages of slow-wave sleep, the third of which is called deep sleep. This means that stages 1 and 2, sometimes referred to as “light” sleep, are labeled as “main” sleep on your wearable device.
So why didn’t Apple use the same wording as everyone else? In a document about its sleep stage algorithm , the company says they are concerned that people will misunderstand the term “light sleep” if they call it that.
The label “Core” was chosen to avoid possible unintended connotations of the term “light” because stage N2 is the predominant (often accounting for more than 50 percent of nightly sleep), normal, and important aspect of sleep physiology, containing sleep spindles and K-complexes.
In other words, they thought we could assume that “light” sleep is less important than “deep” sleep, so they chose a new, important name to use instead of “light.”
The table on the same page shows this: Stages 1 and 2 without REM sleep fall under Apple’s category of “core” sleep, and stage 3 is “deep” sleep. Here’s how Apple defined it in testing: If the EEG showed a person was in Stage 2 when the watch said they were in the “core,” it was considered a success for the algorithm.
What are the actual stages of sleep?
Let’s take a step back and look at what was known about sleep stages before Apple started renaming them. Current scientific understanding , based on brain wave patterns that can be read using EEG, includes the following steps:
Non-REM stage 1 (N1)
N1 lasts only a few minutes. You are breathing normally. Your body begins to relax and your brain waves begin to look different than when you are awake. This will be considered part of your “light” sleep.
Non-REM stage 2 (N2)
N2, which is also generally considered “light” sleep, accounts for about half of your sleep time. This stage includes bursts of brain activity called sleep spindles and characteristic brain wave patterns called K-complexes . (This is what was mentioned in the Apple document above.) It is believed that during this stage of sleep we consolidate our memories. Interesting fact: if you grind your teeth in your sleep, it is mainly at this stage.
Non-REM stage 3 (N3) –
N3 is often referred to as “deep” sleep, and this stage accounts for about a quarter of your night. It has the slowest brain waves, which is why it is sometimes called “slow wave sleep.” It’s difficult to wake someone up at this stage, and if you succeed, they’ll be lethargic for a while afterwards. Most of the body’s recovery usually occurs during this stage, including muscle repair, bone growth in children, and strengthening of the immune system. As we age, we spend less time in N3 and more time in N2.
(There was an older classification that divided the deepest sleep into a separate stage, calling it stage 4 of slow-wave sleep, but the deepest part is now considered simply part of stage 3.)
REM sleep
REM sleep is so named because it is where rapid eye movement occurs. Your body is temporarily paralyzed except for your eyes and breathing muscles. This is the stage best known for dreaming (although dreams can occur in other stages).
The brain waves of a person in REM sleep are very similar to those of a person who is awake, which is why some sleep tracking apps show blocks of REM sleep occurring at the top of the graph, near wakefulness. We usually don’t enter REM sleep until we’ve gone through the other stages, and we cycle through these stages throughout the night. Typically, REM sleep is quite short at the beginning of the night and gets longer with each cycle.
Other uses of the term “core sleep”
I really wish Apple would choose a different term because the phrase “core sleep” has been used in other ways. It either does not refer to the stages of sleep at all, or, if it is related to the stages of sleep, it is used to refer to the stages of deep sleep.
In the 1980s, sleep researcher James Horne proposed that the first few sleep cycles (occupying perhaps the first five hours of the night) constitute the “core” sleep that we all need to function. The rest of the night is “optional” sleep that we should ideally get every night, but skipping it every now and then isn’t that big of a deal. He described this in a 1988 book called Why We Sleep (no relation to another author’s 2017 book), but you can see his earlier article on the topic here . In this article he uses the terms “obligatory” and “optional” sleep, and later moved to basic/optional terminology.
You’ll also find people using the phrase “core sleep” to mean everything other than light sleep. For example, this article on how sleep changes with age compares the results to Horne’s definition of core sleep. However, they describe core sleep as consisting primarily of stages N3-N4 (in other words, N3, as described above).
This is where somehow the idea emerged on the internet that N3 and REM are considered “primary” sleep. I don’t know how this happened and I don’t see it when I look for scientific literature. I see this in the question “What is a core dream?” trash articles on the websites of companies selling weighted blankets and melatonin gummies.
One final, controversial definition: the phrase “core sleep” is also used by people who suffer from polyphasic sleep. This is the idea that you can replace a full night’s sleep with a few naps during the day, something that biohackers keep trying to implement, although it never succeeds . They use the term pretty straightforwardly: if your night’s sleep lasts longer than other types of sleep, it’s your “primary sleep.” Honestly, that’s a fair use of the word. I’ll allow it.
So to recap: Core sleep, if you’re sleeping, is the longest block of sleep you can get during the day. For scientists studying sleep deprivation, core sleep is a hypothesis about what part of the night’s sleep is most important. But if you’re just here because you’re wondering what your Apple sleep app means by “core sleep,” it means stages N1-N2, or light sleep.