Buzz
Let's dive into one of the more perplexing terms you might encounter on your fitness tracker—especially if you use an Apple Watch. Alongside REM sleep, which you may already be familiar with, and 'deep' sleep, a term that seems straightforward, you’ll find the term 'core sleep.' If you search for what core sleep means online, you’ll encounter a definition that’s quite the opposite of how Apple uses the term. Let’s break this down.
The confusion originates from the fact that 'core sleep' has been interpreted in various ways in scientific studies. Most notably, it’s important to note that it’s not an officially recognized sleep stage. However, Apple redefined the sleep stages detected by its watch, and named one of them 'core sleep.' This term, however, doesn’t correspond to any of the traditional definitions of core sleep in scientific literature.
'Core sleep' as defined by the Apple Watch is essentially equivalent to light sleep.
Let me give you a simple breakdown of what you're seeing when you check your Apple sleep data.
Your Apple Watch attempts to estimate, primarily based on your movements, which sleep stage you're in. (To accurately measure sleep stages, a more advanced procedure like a sleep study with equipment such as an electroencephalogram would be necessary. The watch is simply doing the best it can with the available data.)
According to Apple, their watch can differentiate between four distinct states:
Awake
Light (“core”) sleep
Deep sleep
REM sleep
These stages roughly match the sleep phases that neuroscientists observe through polysomnography, which involves connecting you to an electroencephalogram, or EEG (the device where wires are attached to your head). Scientists have identified three stages of non-REM sleep, with the third being categorized as deep sleep. This means that stages 1 and 2, often referred to as 'light' sleep, are what your device labels as 'core' sleep.
In other words, Apple's concept of 'core sleep' aligns perfectly with the scientific definition of 'light sleep,' also known as N2 sleep. (More on that in a moment.)
So, why didn't Apple stick with the standard terminology? According to a document on their sleep stage algorithm, the company expressed concern that the term 'light sleep' might lead to misunderstandings.
The term 'Core' was chosen to avoid any unintended negative connotations associated with 'light,' as the N2 stage is a predominant, normal, and vital part of sleep physiology, often comprising over 50 percent of a night’s sleep. It contains critical features like sleep spindles and K-complexes.
In essence, Apple feared that labeling it 'light' sleep might imply it was less significant than 'deep' sleep. So, they opted for a new, more authoritative-sounding term in place of 'light.'
A chart on the same page illustrates this: non-REM stages 1 and 2 are categorized as 'core' sleep on the Apple watch, while stage 3 is considered 'deep' sleep. During testing, if an EEG showed someone in stage 2 while the watch indicated 'core' sleep, it was counted as a win for the algorithm.
What are the distinct stages of sleep, and where does core sleep fit into this framework?
Let's take a step back and examine the previous understanding of sleep stages, prior to Apple's renaming of them. The current scientific consensus, based on brain wave patterns detectable through an EEG, defines these stages:
Non-REM stage 1 (N1)
Stage N1 lasts only a few minutes. During this time, your breathing remains steady, your body starts to relax, and your brain waves gradually begin to differ from those when you're awake. This stage is considered 'light' sleep, and Apple Watch categorizes it as part of the core sleep stage.
Non-REM stage 2 (N2)
Typically considered 'light' sleep, N2 represents roughly half of your total sleep duration. During this stage, bursts of brain activity, known as sleep spindles, and distinctive brainwave patterns referred to as K complexes occur (as mentioned in the Apple document). It's believed that this stage plays a key role in memory consolidation. Fun fact: if you grind your teeth while asleep, it's likely happening during this phase. Additionally, N2 accounts for the majority of what Apple reports as core sleep.
Non-REM stage 3 (N3)
Often referred to as 'deep' sleep, N3 comprises about a quarter of your sleep cycle. Characterized by the slowest brain waves, it's also known as 'slow wave sleep.' It’s especially difficult to wake someone from this stage, and if you do, they’ll likely feel groggy for a while. This is the phase where significant body repair occurs, including muscle recovery, bone growth in children, and strengthening of the immune system. As we age, we spend less time in N3 and more in N2.
An older classification system once designated the deepest sleep as its own stage, calling it non-REM stage 4. However, this deepest part of sleep is now considered part of stage 3.
REM sleep
REM sleep gets its name due to the rapid movement of the eyes that occurs during this phase. During REM sleep, your body temporarily becomes paralyzed, with the exception of your eyes and the muscles involved in breathing. This is the stage most commonly associated with dreaming, though dreams can also occur during other sleep stages.
The brainwaves during REM sleep resemble those of a person who is awake, which is why some sleep-tracking apps show REM blocks near the wakefulness section of their graphs. REM sleep typically occurs after progressing through the other stages of sleep. Initially, REM sleep is relatively short, but it lengthens with each sleep cycle throughout the night.
How much core sleep do I need?
According to Apple's definition, core sleep is considered the same as light sleep, and it’s normal for nearly half of your sleep to be in this stage. Sleep scientists offer a general breakdown (though exact needs may differ from person to person and night to night):
N1 (very light sleep): Around 5% of your total sleep time (just a few minutes).
N2 (light or 'core' sleep): Roughly 45%, which is about four hours if you usually sleep for eight hours.
N3 (deep sleep): About 25%, approximately two hours if you typically sleep for eight hours.
REM: Also about 25%, so around two hours as well.
How to increase your core sleep
If your Apple Watch reports that you're getting less core sleep than the guidelines suggest, you may wonder how to increase your core (or light) sleep. However, it's important to remember that wearables aren't always precise in determining exactly which sleep stage you're in. They tend to be reasonably accurate in distinguishing between being awake or asleep, so they can be helpful in tracking total sleep duration. But don’t base any changes in your routine solely on the specific sleep stage data. The algorithm can sometimes misclassify light sleep as deep sleep or vice versa.
With that in mind, the most effective way to boost your core sleep is to improve your overall sleep quality. Start by following this basic sleep hygiene checklist. Some of the key recommendations include:
Create a bedtime routine that allows for at least 30 minutes of wind-down time, where you focus on relaxing activities.
Establish a regular wake-up time.
Avoid looking at screens right before bed.
Ensure your bedroom is dark and cool.
Refrain from consuming alcohol or caffeine in the evening.
By enhancing the overall quality of your sleep, you will improve all of your sleep stages, regardless of whether your Apple Watch can distinguish them accurately.
Other interpretations of the term 'core sleep'
I really wish Apple had selected a different term, as 'core sleep' has been used in various contexts. In some cases, it doesn't even refer to a specific sleep stage, while in others, it has been used to describe deep sleep stages.
Back in the 1980s, sleep researcher James Horne suggested that the first few sleep cycles—spanning about the first five hours of sleep—make up the 'core' sleep necessary for proper functioning. The remainder of the night is considered 'optional' sleep, which, although ideal to get, isn't critical if occasionally missed. He explained this concept in his 1988 book, Why We Sleep (which is unrelated to the 2017 book of the same title by a different author). His earlier paper on the subject, which you can find here, uses the terms 'obligatory' and 'facultative' sleep, before switching to the core/optional terminology later on.
Some individuals use 'core sleep' to refer to everything except light sleep. For instance, this paper on how sleep changes with age discusses their findings by comparing them to Horne’s core sleep definition. In this context, core sleep is predominantly made up of stages N3 and N4, which essentially refers to N3 as mentioned earlier.
Somehow, the internet has come to associate both N3 and REM sleep with 'core' sleep. I'm not sure how this misunderstanding arose, as I don’t see any scientific references supporting it. However, this idea can be found in 'what is core sleep?' articles on websites selling products like weighted blankets and melatonin gummies.
In one last, somewhat contradictory usage, the term 'core sleep' is also employed by proponents of polyphasic sleep. This approach suggests that instead of a full night’s rest, several naps throughout the day can suffice—a concept that biohackers continue to experiment with, despite it never truly working. In this context, the definition is quite simple: if you take a nighttime nap that is longer than the others, that’s your 'core sleep.' Honestly, it’s a reasonable interpretation, and I’ll accept it.
To sum it all up: For those who nap, core sleep is the longest sleep block of the day. For scientists studying sleep deprivation, core sleep is a theory about which part of the night’s sleep holds the most importance. But if you’re here because you were curious about what 'core sleep' means in your Apple sleep app, it refers to stages N1 and N2, or light sleep.
