Here’s What “core Sleep” Really Means, According to Your Apple Watch Data.
Let’s talk about one of the most confusing terms you might encounter on your fitness tracker—namely, the Apple Watch . Alongside rapid eye movement (REM) sleep, which you’ve probably heard of, and deep sleep, whose name seems self-explanatory, there’s “core” sleep. And if you search or ask an AI what “core” sleep means, you’ll often get a definition completely different from how Apple uses the term. So let’s clear this up.
On Apple Watch, “core sleep” is another name for light sleep, which scientists also refer to as stages N1-N2. It’s not a type of deep sleep and is unrelated to REM sleep. However, in scientific literature, “core sleep” isn’t a sleep stage at all. It can refer to a portion of the night that includes both deep and light sleep, and in other cases, it can be used to refer to the longest or most significant stretch of time spent in bed. There are several other definitions, which I’ll discuss below. But first, since you’re likely here because you’ve seen this term in Apple Health, let’s talk about how Apple uses it.
On Apple Watch, “core sleep” is the same as light sleep.
Let me give you a simple explanation of what you see when you look at sleep data on your Apple Watch . Your Apple Watch tries to determine, primarily based on your movements, when you’re in each stage of sleep. (To accurately determine sleep stages, a sleep study using more sophisticated equipment, such as an electroencephalogram, would be required. The watch simply does the best it can with the data it has.)
Apple says its watch can distinguish between four different states:
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Awake
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Light (“main”) sleep
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Deep sleep
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REM sleep phase
These categories roughly correspond to the sleep stages that neuroscientists can observe using polysomnography, which involves hooking you up to an electroencephalogram, or EEG. (This is where they attach wires to your head.) Scientists distinguish three stages of non-REM sleep, the third of which is described as deep sleep. This means that stages 1 and 2, sometimes called “light” sleep, are labeled by your sleep wearable as “core” sleep.
In other words, Apple’s definition of “core sleep” is identical to the definition of “light sleep” given by scientists. It’s also known as N2 sleep. (More on that later.) So why didn’t Apple use the same wording as everyone else? In a paper on its sleep stage algorithm, the company states that it was concerned people would misunderstand the term “light sleep” if it called it that.
The text states: “The name Core was chosen to avoid possible unintended associations with the term ‘light sleep,’ as stage N2 is the predominant (often accounting for more than 50 percent of nighttime sleep), normal, and important aspect of sleep physiology, including sleep spindles and K-complexes.” In other words, Apple thought we might assume that ‘light’ sleep is less important than ‘deep’ sleep, so it chose a new, seemingly important name instead of ‘light.’
The same page includes a diagram illustrating this: stages 1 and 2, which are not part of the rapid eye movement (REM) phase, are classified by Apple as “core” sleep, while stage 3 is considered “deep” sleep. This is how Apple determined this during testing: if the EEG showed a person in stage 2 while the watch showed them in “core” sleep, the algorithm considered it a success.
What stages of sleep are known, and where does primary sleep fit into this list?
Let’s revisit what was known about sleep stages before Apple started renaming them. Current scientific understanding , based on brainwave patterns that can be read using an EEG, includes the following stages:
Stage 1 non-REM sleep (N1)
Stage N1 lasts only a few minutes. You breathe normally. Your body begins to relax, and your brain waves begin to differ from those observed when you’re awake. This is considered part of your “light” sleep phase. Apple Watch considers this part of your main sleep stage.
Stage 2 non-REM sleep (N2)
This stage, commonly considered “light” sleep, accounts for approximately half of your sleep time. During this stage, bursts of brain activity called sleep spindles and characteristic brainwave patterns called K-complexes are observed. (This is what the Apple document mentioned above describes.) It is believed that this stage of sleep is where memory consolidation occurs. Fun fact: if you grind your teeth in your sleep, it mostly happens during this stage. This stage accounts for the majority of what Apple calls your core sleep.
Stage 3 of non-REM sleep (N3)
Stage N3 is often called “deep” sleep and accounts for about a quarter of the night. It is characterized by the slowest brain waves, which is why it is sometimes called “slow-wave sleep.” It is difficult to awaken a person from this stage, and even if you do, they will feel sluggish for some time. It is during this stage that the body’s greatest restoration occurs, including muscle repair, bone growth in children, and a strengthening of the immune system. As we age, we spend less time in stage N3 and more time in stage N2.
(There was an older classification that classified the deepest sleep as a separate stage called non-REM stage 4, but this deepest portion is now considered part of stage 3 only.)
REM sleep phase
Rapid eye movement (REM) sleep gets its name from the rapid eye movement that occurs during this phase. The body is temporarily paralyzed, except for the eyes and respiratory muscles. This stage is best known for dreaming (although dreams can occur during other stages as well).
A person’s brainwaves during rapid eye movement (REM) sleep are very similar to those of a waking person, which is why some sleep tracking apps show REM sleep blocks at the top of the graph, closer to wakefulness. We typically enter REM sleep only after passing through other stages, cycling through these stages throughout the night. Typically, REM sleep is relatively brief at the beginning of the night and increases in duration with each cycle.
How much time do I need for a good night’s sleep?
According to Apple’s definition, where core sleep and light sleep overlap, it’s normal for nearly half of your sleep to be spent in core sleep. Sleep experts provide a rough breakdown (though the exact numbers can vary from person to person, and your needs aren’t always the same every night):
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N1 (very light sleep): about 5% of the total time (only a few minutes)
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N2 (light or “core” sleep): about 45%, or just under four hours if you usually sleep eight hours.
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N3 (deep sleep): About 25%, which is about two hours if you usually sleep eight hours.
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REM: About 25%, which is about two hours.
How to improve sleep quality
If your Apple Watch shows that you’re getting less core (or light) sleep than I mentioned above, you may be wondering how to increase your core (or light) sleep. However, before taking any action, it’s important to know that wearables aren’t very good at determining your exact sleep stage. They’re usually (but not always!) fairly accurate at determining whether you’re asleep or awake, so they can be useful for determining whether you’ve slept six or eight hours. However, I wouldn’t make any changes to your routine based on specific sleep stage readings. The algorithm can easily misclassify some of your light sleep as deep sleep, or vice versa.
However, the best way to improve your sleep quality is to get enough sleep overall. Start with this basic sleep hygiene checklist . Among the most important points:
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Develop a pre-bed ritual that includes at least 30 minutes of rest, during which you try to do something relaxing.
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Maintain a consistent wake-up time.
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Avoid looking at screens immediately before bed.
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The bedroom should be dark and cool.
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Avoid drinking alcohol and caffeine in the evening.
Improving your overall sleep quality will improve all stages of sleep, whether your Apple Watch can detect them or not.
Other uses of the term “core sleep”
I’m really sorry Apple didn’t choose a different term, because the term “core sleep” has been used in other ways. It either doesn’t refer to sleep stages at all, or, if it does refer to sleep stages, it’s used to refer to deep sleep stages.
In the 1980s, sleep scientist James Horne proposed that the first few sleep cycles (perhaps the first five hours of the night) constitute “core” sleep, the sleep we need to function normally. The rest of the night is “optional” sleep, which we should ideally get every night, but skipping it occasionally isn’t a big deal. He described this idea in a 1988 book called “Why We Sleep” (unrelated to the 2017 book by another author), but you can read his earlier article on the topic here . In that article, he uses the terms “obligatory” and “elective” sleep, and later shifted to the “core”/”optional” sleep terminology.
You might also encounter the term “core sleep” used to refer to everything other than light sleep. For example, in this article about how sleep changes with age, the authors compare their findings in terms of sleep stages to Horne’s definition of core sleep. They describe core sleep as consisting primarily of deep sleep stages N3-N4 (in other words, N3, as described above).
Somewhere on the internet, the idea emerged that N3 and REM stages are considered the “primary” sleep stages. I don’t know how this came about, and I can’t find it in the scientific literature. I’ve encountered it in meaningless articles on the websites of companies selling weighted blankets and melatonin gummies, asking “What is primary sleep?” Meanwhile, Google’s AI-powered search mashes up all these definitions to come up with something that’s currently completely meaningless. But who knows, maybe by the time you read this, things will have changed.
As a final, controversial definition, the phrase “core sleep” is also used by people who are into polyphasic sleep. This is the idea that you can replace a full night’s sleep with several naps during the day, something biohackers constantly try to achieve, though it never quite works . They use the term quite bluntly: if you have a nighttime nap that lasts longer than the others, that’s your “core sleep.” Frankly, this is a perfectly fair use of the word. I admit it.
So, to summarize: core sleep, if you’re a napper, is the longest stretch of sleep you get during the day. For sleep deprivation researchers, core sleep is a hypothesis about which part of nighttime sleep is most important. But if you’re just curious about what Apple Health or the sleep app on your Apple Watch means by “core sleep,” it refers to stages N1-N2, or light sleep.