Understanding Core Sleep: What Your Apple Watch Thinks It Is


Companions, let’s converse about one of the most bewildering terms you’ll encounter on your fitness monitor—specifically your Apple Watch. Adjacent to REM sleep, which you’ve likely heard of, and “deep” sleep, which appears self explanatory, there’s “core” sleep. And if you google what core sleep signifies, you’ll receive a definition that is completely contrary from how Apple employs the term. So, let’s dissect it.

The crux of the uncertainty lies in the reality that the term “core sleep” has been applied in the scientific writings to signify a few distinct things. Significantly, it’s not a recognized sleep phase. Apple, alternatively, elected to rename the sleep phases its watch can detect, and dubbed one of them “core sleep”—but it has no relation to any of the preceding common uses of the term.

Apple designates light sleep as “core sleep”

Permit me to provide you with a simple explanation of what you’re witnessing when you inspect your Apple sleep information.

Your Apple Watch endeavors to estimate, mainly through your motions, when you’re in each phase of sleep. (To fully understand your sleep stages would necessitate a sleep study with more advanced equipment. The watch is simply doing its best with the data it possesses.)

Apple asserts that their watch can discern the difference between four distinct states:

  • Awake

  • Light (“core”) sleep

  • Deep sleep

  • REM sleep

These classifications approximately correspond to the sleep phases that neuroscientists can observe with polysomnography, which encompasses connecting you up to an electroencephalogram, or EEG. (That’s the contraption where they connect wires to your head). Scientists acknowledge three phases of non-REM sleep, with the third being delineated as deep sleep. This implies that phases 1 and 2, which are every so often denoted as “light” sleep, are being designated as “core” sleep by your wearable.

Thus, why didn’t Apple use the identical wordings as everybody else? The corporation stated in a document on their sleep stage algorithm that they were apprehensive people might err in understanding the term “light sleep” if they labeled it as such.

The label Core was selected to evade conceivable unintended implications of the term light, because the N2 stage is predominant (often constituting more than 50 percent of a night’s sleep), normal, and an important aspect of sleep physiology, encompassing sleep spindles and K-complexes.

In other terms, they reckoned we might presume that “light” sleep is less crucial than “deep” sleep, so they picked a new, significant-sounding name to utilize instead of “light.”

A graph on the identical page lays it out: non-REM phases 1 and 2 are classified under the Apple category of “core” sleep, while phase 3 is “deep” sleep. That’s how Apple defined it through testing: If an EEG denoted a person was in phase 2 when the watch denoted they were in “core,” that was recognized as a triumph for the algorithm.

What are the genuine sleep phases?

Lets retreat to evaluate what was recognized regarding sleep phases ahead of Apple commencing renaming them. The current scientific understanding, which is grounded on brain wave patterns that can be deciphered with an EEG, encompasses these phases:

Non-REM phase 1 (N1)

N1 exclusively lasts a couple of minutes. You’re breathing routinely. Your body is in the initial phases of unwinding, and your brain waves commence to appear distinct compared to when you’re awake. This would be contemplated as part of your “light” sleep.

Non-REM phase 2 (N2)

Additionally usually accounted as “light” sleep, N2 constitutes approximately half of your sleep duration. This phase encompasses spikes of brain activity termed sleep spindles, and distinct brainwave patterns dubbed K complexes. This phase of sleep is believed to be when we consolidate our memories. An interesting detail: if you clench your teeth in your sleep, it will largely happen in this phase.

Non-REM phase 3 (N3)

N3 is frequently referred to as “deep” sleep, and this phase takes up about a quarter of your night. It features the slowest brain waves, so it’s sometimes denoted as “slow wave sleep.” It’s hard to rouse someone from this phase, and if you manage to, they’ll feel groggy for a brief period thereafter. This is the stage where most body restoration typically occurs, including muscle recuperation, bone growth in children, and fortifying the immune system.

(There was an old-fashioned classification that segregated the deepest sleep into its separate stage, labeling it non-REM phase 4, however currently, that deepest portion is simply regarded as part of phase 3.)

REM sleep

REM sleep is dubbed so because this is where we undergo Rapid Eye Movement. Your body is temporarily immobilized, except for the eyes and your breathing muscles. This is the stage best recognized for dreaming. The brain waves of a person in REM sleep are strikingly similar to those of a person who is awake. This is the reason why some sleep-tracking apps depict blocks of REM as transpiring near the pinnacle of the graph, proximate to wakefulness. We usually don’t enter REM sleep until we’ve passed through the other stages, and we alternate through these stages all night. Usually, REM sleep is relatively brief at the commencement of the night, and elongates with each cycle.

Other methods individuals utilize the term “core sleep”

I truly wish Apple had picked a different term, due to the reality that the expression “core sleep” has been employed in other ways. It either doesn’t allude to a sleep stage at all, or if it is connected with sleep stages, it’s employed to refer to deep sleep stages.

In the 1980’s, sleep researcher James Horne put forth the notion that your initial few sleep cycles (occupying perhaps the first five hours of the night) constitute the “core” sleep required to operate efficiently. The remaining part of the night is “optional” sleep, which ideally we’d still acquire nightly, but it’s not imperative to abstain from now and then. He detailed this in a 1988 book dubbed Why We Sleep (unrelated to the 2017 book by another author) but you can explore his earlier paper on the topic here. He employs the terms “obligatory” and “facultative” sleep in that paper, and shifted to the core/optional terminology afterward.

You’ll also encounter individuals employing the phrase “core sleep” to denote everything outside of light sleep. For instance, this paper on how sleep transforms with age compares their findings in terms of sleep stages with Horne’s notion of core sleep. In doing so, they portray core sleep as mainly comprising phases N3-N4 (meaning N3 as depicted above).

From there, by some means the internet has garnered the notion that N3 and REM are deemed “core” sleep. I don’t know how that transpired, and I don’t encounter it when I search the scientific literature. I do observe it on “what is core sleep?” spurious articles on the websites of corporations peddling weighted blankets and melatonin gummies.

For one concluding, conflicting definition, the phrase “core sleep” is also employed by individuals enthusiastic about polyphasic sleep. This is the notion that you can substitute a complete night’s sleep with multiple naps during the day, something that biohacker types keep attempting to actualize, even though it never culminates. They use the term pretty straightforwardly: If you have a nighttime nap that is lengthier than your other naps, that’s your “core sleep.” Truthfully, that’s a valid use of the word. I’ll permit it.

Therefore, in conclusion: Core sleep, if you’re a napper, is the lengthiest block of sleep you receive during a day. Core sleep, to scientists who investigate sleep deprivation, is a hypothesis about which segment of a night’s sleep is the most crucial. But if you’re simply here because you were pondering what your Apple sleep app conveys by “core sleep,” it indicates phases N1-N2, or light sleep.

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