Medically reviewed by Jared Meacham, PhD, RD, CSCS and Nesochi Okeke-Igbokwe, MD, MS

“Core sleep” is a term Apple Watch uses to describe the early sleep stages, where you’re drifting or getting initial light sleep. Understandably, you may be wondering how much “core sleep” you need. But the term is a little confusing. We’ll explain.

We all know at a basic level that a good chunk of sleep time (seven to nine hours for adults) is crucial to our health. But what many people may not realize is that not all our hours of shuteye are the same. Instead, our sleep has structure to it called sleep architecture.

Sleep architecture is a bit of a weird term, as if you’ve got Frank Lloyd Wright designing you an open floor plan while you rest. That would be nice. But the term actually describes the various cycles, phases, and stages that we go through over the course of several hours in a solid sleep session.

The topic of sleep can get even more confusing with terms such as “core sleep.” It’s a term Apple adopted to denote the stages of drifting off and light sleep. But admittedly it kind of sounds like core sleep means crucial sleep, like it’s the most important zzz’s you’ll get over the course of the night.

The confusion comes in because Apple’s use of “core sleep” doesn’t align with the original scientific definition. Only older research, (pre-2000s) uses the term, but to denote a different stage: deep sleep.

In this article, we explore why “core sleep” is a confusing term, how Apple uses it, what it really means for you, how BetterSleep can help, and more.

What is core sleep?

That’s a complicated question to answer. Here’s why. First, it’s not a scientific term, and the definition differs, depending on where you look.

• Older science: A 1992 journal article describes it as slow wave sleep which is composed of stage 3 and 4 of non-REM sleep, although newer scientific literature combines these stages and labels them as stage 3. Slow wave sleep is considered your deepest sleep stage, accounting for about 25% of your total sleep time. But generally research doesn’t use the term core sleep.
• Apple Watch: Apple uses “core sleep” to categorize the light stages of sleep. The first two sleep phases you go through are on the lighter side. But these stages account for about 50% of your sleep. The company says, “The label ‘core’ was chosen to avoid possible unintended implications of the term light.” If Apple Watch labeled this time as “light sleep,” users might be worried they aren’t getting good quality sleep.
• General wellness articles: Since Apple launched its sleep-tracking option in 2020, the media has also adopted the term “core sleep.” Still, it’s misleading since it doesn’t align with how science discusses sleep.

Are you confused yet? We’re here to make things clearer by explaining sleep architecture, why it matters, and what you need to know in simple terms.

Sleep 101: A Glossary of Terms

Before we delve further into what core sleep is, we have to define sleep architecture and its cycles and stages in basic terms. Remember, that just because you sleep for, say, eight hours doesn’t mean all of that sleep is the same. Sometimes you’re in a lighter sleep stage, other times a deeper one, and sometimes you dream. All of these stages have different purposes.

• Sleep architecture: This is the structure of your sleep, including cycles and stages.
• Sleep cycles: We go through four to six sleep cycles per night, each with two phases.
• Sleep phases: The two sleep phases include non-rapid eye movement (NREM) and rapid eye movement (REM), with four stages across the two.
• Sleep stages: The four stages consist of three NREM one REM, making up various percentages of your total sleep time.
  • NREM 1: ~5%
  • NREM 2: ~45%
  • NREM 3: ~25%
  • REM: ~25%

“The majority of our sleep time is made up of non-REM stage 1 light sleep and non-REM stage 2 sleep, which is a transitional stage between light and deep sleep,” says Chelsie Rohrscheib, PhD, a neuroscientist and sleep head researcher at Wesper. “The word ‘core’ may be confusing, as it implies these are the most essential or restorative stages, which negates the importance of stage 3 slow-wave sleep and REM sleep.”

Understanding sleep cycles

Each of our four to six sleep cycles takes roughly 90 to 110 minutes. Each cycle involves the four different stages before the next cycle begins or you naturally wake up.

To find out how many sleep cycles you need, determine how many hours of sleep help you feel your best.

• seven hours: four cycles
• eight hours: five cycles
• nine hours: six cycles

Understanding sleep stages

Each sleep cycle consists of four stages: Three NREM stages and one REM stage.

NREM stage 1

When you start to drift off, you enter NREM 1. You’re not quite asleep and not quite awake. You know those little muscle twitches that make you accidentally kick your bed partner or that sensation of falling into your mattress that jolts you awake? Those are called hypnic jerks, which are common in NREM 1. This stage lasts from one to five minutes and accounts for about 5% of your total sleep time.

Need to ease yourself into NREM stage 1? Try BetterSleep’s relaxation and meditation options.

NREM stage 2

This stage is a bit deeper than NREM 1. Your body temperature and heart rate lower. Your brain engages in neuroplasticity, the growth and reorganization of its neural network.

Key features of NREM 2 sleep are:

• Sleep spindles: During this stage, you experience sleep spindles, when neurons have bursts of firing. Sleep spindles aid with long-term memory consolidation related to procedural and declarative memory.
  • Procedural memory consolidation: If you learn a new skill or enhance the practice of another, your brain will store the fresh knowledge in your long-term procedural memory. Examples include learning a new language, playing an instrument, or performing the steps to a new TikTok dance.
  • Declarative memory consolidation: Declarative memory includes remembering information (semantic memory), such as the key points of a presentation you have to give or facts you need to know for a test. It also includes episodic memory, or the recollection you have of past events, like your first kiss or where you went on your last family vacation.
• K-complexes: In this stage, you also experience k-complexes, which are long delta waves that aid with maintaining sleep and memory consolidation.

In your first sleep cycle, you’ll spend about 25 minutes in NREM 2. As the night progresses, you’ll spend increasing time in this stage. It accounts for about 45% of your total sleep time.

NREM stage 3

This stage is your deepest level of sleep. Your heart rate and breath rate slow the most. Waking becomes more difficult when compared to other stages. If someone needs to wake you, they might have to try more than once. Likewise, if you must awaken during this stage, such as from an alarm, you might feel sluggish or foggy for a bit.

“NREM 3—deep (slow wave) sleep—is the stage where most of our restorative processes occur, including brain maintenance,” Dr. Rohrscheib explains. “This includes synaptic homeostasis, which is a technical term for balancing the activity of our neurons, brain waste clearance, hormone release, and neuronal repair.”

Key features of NREM stage 3 sleep are:

• Daily playback: Your brain replays what you learned during the day and commits it to your long-term memory. Meanwhile, your brain makes room for new content—almost like a closet cleanout, getting rid of what you no longer need or use.
• Physical repair: Your body needs NREM 3 to recover and repair itself. Did you engage in a workout? NREM 3 will help your body adapt and come back stronger at the gym, track, or court.
• Immune system support: This stage ramps up your body’s production of cytokines (immune system proteins) to help fight off any pathogens.
• Bedwetting, night terrors, and sleepwalking: These issues typically occur during NREM 3. Bedwetting may mean you’re not recognizing your bladder’s signals during this deep-sleep stage, for example. And night terrors can occur from a sudden awakening (though not fully) from this deep stage without having a transition.

In your first sleep cycle, you’ll spend about 50 minutes in NREM 3, but the time you spend in this stage decreases with each additional cycle. NREM 3 generally accounts for 25% of your sleep.

To minimize any potential disruptions to your sleep, try some white noise to block out city or bedmate sounds.

REM

After NREM 3, you enter REM, which—although you are sleeping—is not really a restful stage.

• Rapid eye movement: REM gets its name because your eyes move rapidly, though researchers are unclear why. Likely, it has to do with visual memory consolidation and aiding our comprehension of visual information.
• Dreaming: You dream during REM sleep, which is also important for memory consolidation and stabilization, problem-solving, emotional processing, and your stress response.
• Paralysis: You also lack muscle tone and are temporarily paralyzed so that you don’t act out your dreams. If you’ve ever awoken to the sensation of, or even seeing, a malevolent figure hovering over you and you can’t move, you are actually experiencing a transition glitch called a “hypnagogic hallucination.” Understandably, this can be terrifying. Your body is still paralyzed from REM, but you’re caught between REM and a fully awakened state with your mind playing tricks.
• Transition: REM serves as a buffer or transition from deep NREM 3 before waking or starting a new sleep cycle. Waking from NREM 3 can be too jarring, so REM helps you ease into an awakened state.

Your first REM stage will be relatively short, about 10 minutes. But as you get closer to waking, you’ll spend about an hour in REM sleep to prepare you for the day ahead. REM sleep accounts for about 25% of your total sleep time.

Is Core Sleep Good—And Why Does It Matter?

Again, core sleep has several definitions, none of them consistent. What good quality sleep ultimately comes down to is whether you’re getting adequate sleep time that includes all the cycles you need and whether you’re spending enough time in the sleep stages noted above.

Keep in mind that sleep stages change in duration with each cycle. Not getting enough sleep, or experiencing significant sleep disruptions, can potentially reduce not only your quantity but also the quality you’re getting. This decreases the benefits you experience with each stage and cycle.

“All stages are crucial for maintaining health and [to] achieve different biological functions,” Dr. Rohrscheib says. “Too much or too little of any sleep stage can be detrimental to health.”

How Trackers Define Core Sleep

You may be using a sleep tracker to monitor how much time you’re spending in the various stages of sleep, and this can make understanding core sleep even trickier. Again, core sleep isn’t a scientific term—though older research referred to it as NREM 3.

• Of the popular sleep trackers on the market, Apple is responsible for getting the term “core sleep” into the zeitgeist, labeling core sleep as NREM 1 and NREM 2.
• Fitbit, Oura, Garmin, Samsung, and BetterSleep don’t define core sleep; instead they label sleep stages as light, deep, and REM.
• Whoop doesn’t define core sleep either, but it defines NREM 3 and REM as “restorative sleep,” with the other sleep stages as nonrestorative.

How trackers label sleep stages and what to aim for:

Tracker	Terminology used	Sleep stages and definitions	Recommended amount
BetterSleep	Light/Deep/REM	Light = NREM 1 + NREM 2; Deep = NREM 3	Light: 50% Deep: 25% REM: 25%
Apple	Core sleep	Core Sleep = NREM 1 + NREM 2	Core: 50%
Oura	Light/Deep/REM	Light = NREM 1 + NREM 2; Deep = NREM 3	Light: 50% Deep: 25% REM: 25%
Garmin	Light/Deep/REM	Light = NREM 1 + NREM 2; Deep = NREM 3	Light: 50% Deep: 25% REM: 25%
Fitbit	Light/Deep/REM	Light = NREM 1 + NREM 2; Deep = NREM 3	Light: 50% Deep: 25% REM: 25%
Samsung	Light/Deep/REM	Light = NREM 1 + NREM 2; Deep = NREM 3	Light: 50% Deep: 25% REM: 25%
Whoop	Restorative/Nonrestorative	Restorative = NREM 3 + REM; Nonrestorative = NREM 1 + NREM 2	Restorative: 50% Nonrestorative: 50%

How trackers measure core sleep

Now that you understand sleep architecture and what your sleep tracker tells you about your sleep, you may be wondering: How does it even know my sleep stages in the first place? And is it right? Great questions.

Trackers use a mix of motion sensing and heart rate signals to estimate your sleep stages. Here’s a look at the various technologies.

• Accelerometer: Motion sensing detects movement patterns to estimate sleep and awake states.
• Photoplethysmography (PPG): PPG uses light shined on skin to measure blood flow, which then calculates your heart rate (HR) and your heart rate variability (HRV) to determine stages.
• Other sensors: Several other metrics can inform sleep stages. These include:
  • Body temperature
  • Blood oxygen level
  • Respiration rate
  • Microphone (to detect breathing and snoring)

Ultimately, sleep trackers vary in their accuracy. They are generally good at estimating sleep vs awake time with greater than 95% sensitivity. But they are less accurate for estimating your sleep stages, with sensitivity ranging from 50% to 86%.

How Long Should You Be in Core Sleep?

The answer to this question depends on the definition of core sleep. Since Apple is the only tracker using this term, and Apple defines core sleep as NREM 1 and NREM 2, we can look at how much time science says you should spend in these specific sleep stages over the course of a night or full sleep session. Roughly you should spend about 5% of your sleep time in NREM 1 and 45% in NREM 2 for an estimate of 50% of your total sleep.

How much time is that in hours and minutes? That, of course, depends on the duration of your total sleep session. Someone getting eight hours of sleep total would spend about four hours across NREM 1 and NREM 2 (with about 3.5 hours in NREM 2).

Core vs. Deep Sleep

What is the difference between core sleep and deep sleep? Again, core sleep is not a scientific term. Apple uses it to bundle NREM 1 and NREM 2, your lightest sleep stages together, while NREM 3 is your deepest stage of sleep.

With the caveat that individual sleep architecture varies, you should spend about 50% of your sleep in NREM 1 and NREM 2, which is Apple's combined definition of “core sleep,” and about 25% in NREM 3, which older science calls “core sleep” because of its restorative nature.

How to Improve Your Core Sleep (And Overall Sleep Mix)

Rather than focusing on improving core sleep, which can be a confusing way to assess your overall sleep architecture, you may wish to turn your attention to ways to improve your overall sleep mix.

“It begins with doing all the right things during your waking hours,” says Maha Alatar, MD, the director of sleep medicine at VCU Health in Virginia. She provides the following tips:

• Avoid caffeine after noon, especially if you are sensitive to it.
• Engage in physical activity; but you may wish to avoid strenuous exercise before bedtime. Gentle options may be best.
• Create a wind-down routine to prepare yourself for bed.
• Get outdoors for some natural light, early in the day if you can, which helps set your body’s sleep-wake cycle.
• Avoid screens too close to bedtime; these emit blue light, which can suppress melatonin, the sleep hormone.
• Manage stress, which can affect your sleep.

“Muscle relaxation activities that include progressively relaxing your body from toes to your forehead ease you to sleep and can simply take the anxiety or stress away,” Dr. Alatar adds. “You can also do some deep breathing exercises, as they stimulate the relaxation part of your system. A nice warm shower in the evening is a good idea.”

Some sleep apps can also help you improve your slumber. You can download the BetterSleep app for a free trial.

When to Talk to a Clinician

“You want to see a physician if there is a change in your sleep and basically when your sleep is not satisfying or you are waking up not rested or feeling sleepy and tired during the day,” adds Dr. Alatar.

According to Dr. Rohrscheib, you should see a doctor if you experience the following:

• Your sleep is chronically unrefreshing even if you feel like you achieve enough sleep time.
• You have chronic insomnia that lasts three months or longer.
• You have frequent nighttime awakenings.
• Your poor sleep aligns with other health conditions like hypertension, high blood sugar, or heart arrhythmias.
• Your bed partner notices that you stop breathing during sleep, snore, gasp, or choke—all potential signs of sleep apnea.

“If not treated, [sleep apnea] can lead to vascular health problems, including strokes and heart disease. Unfortunately, it increases in prevalence with middle age and beyond,” Dr. Alatar explains.

The Bottom Line

Hopefully, after reading this article, you have a better understanding of “core sleep.” Ultimately Apple Watch uses the term to note how much sleep you’re getting in the early and light sleep stages. However, all stages of sleep are important. Focus on getting good quality sleep by allotting seven to nine hours and minimizing disruptions, and you’ll be doing your body and brain good.

Key Takeaways

• “Core sleep” is a term Apple Watch uses to describe the early sleep stages of drifting off and getting initial light sleep before slipping into a deeper stage.
• However, “core sleep” is not a scientific term. Instead, researchers discuss sleep quality and quantity in terms of sleep architecture.
• Good sleep architecture consists of four to six sleep cycles with adequate time in the four stages of each cycle.
• If you’re using the Apple Watch, a ballpark goal is to spend 50% of your total sleep time in “core sleep.”
• Looking to get better sleep? Download the BetterSleep app for a free trial.

Frequently Asked Questions (FAQ’s)

How many hours of core sleep is good?

“Core sleep” is not a scientific term. Apple Watch uses the phrasing to bundle NREM 1 and NREM 2 (your lightest sleep stages) together, while NREM 3 is your deepest stage of sleep. You should spend an estimated 50% of your sleep in NREM 1 and NREM 2. So if you get eight hours of sleep, core sleep should account for roughly four hours, though your results may be highly individual.

What does core sleep mean on an Apple Watch?

“Core sleep” is not a scientific term, and it can be misleading. Apple Watch uses the phrasing to describe the combined time spent in NREM 1 (drifting off) and NREM 2 (light sleep). Older scientific studies define core sleep as NREM 3, your deepest stage of sleep.

Is core sleep better than deep sleep?

“Core sleep” is a phrase that Apple Watch uses when referring to the two lightest stages of sleep, and accounts for the most time in the sleep cycle. The other, deeper stages of sleep are shorter in duration, but just as important for health. One is not better than the other.

How many hours of sleep do I need?

The Centers for Disease Control and Prevention (CDC) recommends that adults get seven to nine hours of sleep per night.