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Forget about core sleep on your Apple Watch — this is the sleep metric you really need to pay attention to

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Expert reveals why deep sleep is crucial for recovery, plus how to get more of it

Our sleep tech tester testing the Apple Watch 10
(Image credit: Future)
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If you use an Apple Watch to track your sleep, there's one stage of sleep that you might be confused by — core sleep. What exactly is it and how much do you need?

The Apple Watch 10 offers up in-depth data surrounding how long you’ve slept, how often you wake up, and how your time is divided across different sleep stages.

Ready to track your sleep?

A collage of our sleep tech reviewer testing various sleep trackers in our studio. Laying in bed with white sheets wearing the Oura Ring 4 (top left). Apple Watch 10 with purple strap on wrist displaying sleep stages data (top right). All sleep tracker on arm resting on Withing sleep mat (bottom left). Checking sleep data on mobile wearing Garmin sleep tracking armband on right arm (bottom right).

(Image credit: Future)

Our team of expert tested have tested the best sleep trackers on the market to bring you our guide to the 5 leading options on the market for all budgets.

Within those stages, Apple groups your sleep into awake, REM, core, and deep — labels that might make sense at first glance, but often raise more questions than they answer.

One term in particular tends to confuse people: core sleep. It’s an unusual choice of wording, because most other sleep trackers don’t use it at all.

To complicate things further, some online definitions describe core sleep as something entirely different or even the opposite of Apple's definition.

Below, we'll dig into what core sleep on an Apple Watch actually means, how it fits into your overall sleep cycle, and which sleep stages are the most useful indicators of sleep quality.

What are the stages of sleep?

Before we dive into what core sleep means on your Apple Watch, and which sleep stage you should actually be paying attention to, it’s useful to revisit the four stages of sleep and what they do.

Across a typical night, your body cycles through these stages multiple times, with each one playing a different role in physical recovery, brain function, and overall sleep quality.

N1 (light sleep)

N1 is the lightest stage of sleep and acts as the transition between being awake and fully asleep. Your heart rate begins to slow, muscles relax, and brain activity starts to change.

This stage usually only lasts a few minutes at a time, and most people spend around 5% of the night in N1 sleep. While brief, it’s important because it allows the body to gradually switch off rather than dropping abruptly into deeper sleep stages.

A man lies on his back in bed with his eyes closed and one hand resting on his chest.

(Image credit: Getty Images)

N2 (light sleep)

N2 is still considered light sleep, but it’s where you spend the largest portion of the night. During this stage, your heart rate and breathing continue to slow, body temperature drops, and brain activity becomes more stable.

N2 sleep plays a key role in memory and learning. Most adults spend around 40–55% of their total sleep time in N2, making it a major contributor to overall sleep quality.

N3 (deep sleep)

A couple falling asleep in bed

(Image credit: Getty Images)

N3 is deep sleep, sometimes it’s also called slow-wave sleep, and it’s the most physically restorative stage. This is when the body focuses on tissue repair, muscle growth, immune function, and releasing growth hormone.

Brain activity slows significantly, making this stage harder to wake from. Adults typically spend around 15–25% of the night in deep sleep, with more earlier in the night and less as morning approaches.

REM sleep

REM (rapid eye movement) sleep is when most dreaming occurs and brain activity becomes more active again. This stage is crucial for emotional regulation, creativity, and processing memories.

REM sleep periods get longer as the night goes on, and most adults spend around 20–25% of their total sleep in REM sleep by morning.

What is Core sleep on your Apple Watch?

So where does core sleep fit into the stages above? Somewhat confusingly, core sleep can mean different things depending on who you ask. In some scientific and sleep-medicine contexts, people use 'core sleep' to refer to the most restorative, non-REM portion of the night (often associated with deeper sleep).

Sleep stages on Apple Watch 10

(Image credit: Future)

Apple, however, use the term core sleep differently. On an Apple Watch, core sleep is essentially light sleep, the N1 and N2 stages. Apple has said it chose the label partly to avoid the implication that 'light' sleep is in any way negative.

In reality, core (light) sleep supports learning and memory processes, helps your body settle into deeper stages, and makes up the largest chunk of a typical night. Most people spend roughly 50–60% of their sleep in this stage.

However, if the percentage you're spending in core is higher than that, it could be a sign you’re not spending enough time in deeper, more restorative sleep.

Which sleep stage is most important?

Good quality sleep is dependent on moving through all stages of the sleep cycle in the right proportions, multiple times per night. However, deep sleep is particularly important for immune health, muscle repair, and restoring the body after physical and mental stress.

A woman lies in on her side in bed at night, she is sleeping deeply.

(Image credit: Getty Images)

"Deep sleep significantly contributes to growth and repair by triggering the release of human growth hormone from the pituitary gland, which stimulates the body to rebuild and repair tissues like muscles and bones," Dr. William Lu, a sleep medicine physician, told us during a previous conversation surrounding deep sleep.

That's why kids and teens require more deep sleep, as that's when they're growing. Most adults benefit from around 1.5 to 2 hours of deep sleep per night, though this can vary with age, activity levels, and overall health.

5 ways to get more deep sleep

Experts suggest that 1.5 to 2 hours of deep sleep per night helps us to feel properly rested the next day. If your Apple Watch shows you’re regularly getting less than that, it could mean you need to clock up more deep sleep — here's how.

1. Impose a screentime ban before bed

Blue light from phones, tablets and TVs can suppress melatonin, the hormone that helps signal to your body that it’s time to go to sleep. This can delay falling asleep and reduce the amount of deep sleep that you get.

Ideally, experts recommend avoiding screens for at least an hour before bed. If that feels unrealistic, lowering brightness, using warmer colour settings or switching to just audio content can still help too.

A woman sitting in her bed at night, looking at her phone. She is illuminated by the light from her bedside lamp.

(Image credit: Getty Images)

2. Stick to the '7:1' sleep rule

Going to bed and waking up at roughly the same time each and achieving at least seven hours of sleep per night (AKA, the 7:1 sleep rule) helps regulate your circadian rhythm, which is your internal body clock. When your sleep schedule is consistent, your body moves more efficiently through the different stages of sleep, including deep sleep.

That consistency matters even on weekends, when late nights and lie-ins can quietly undo the rhythm you’ve built during the week.

3. Cut down on alcohol consumption

Alcohol can initially make you feel sleepy, but it disrupts sleep quality once you’re actually asleep. Research shows that even two or three drinks can reduce both deep sleep and REM sleep.

Alcohol changes your sleep architecture, meaning you may still clock eight hours but wake feeling groggy, unfocused, or unrested because your brain didn’t spend enough time in its most restorative stages.

Woman drinking a glass of wine in a white bed wearing pajamas in a dim lit room

(Image credit: Getty Images)

4. Practice a pre-bed brain dump

Nighttime anxiety can be a barrier to getting the deep sleep you need, which makes it difficult to both fall and stay asleep. A tried and tested method on the Tom's Guide sleep team is to practice a pre-bed brain dump to avoid rumination.

In a separate room to one you sleep in (to avoid building a negative association with stress and worry), spend three minutes jotting down what's on your mind. That could be the things you're worrying about, your to-do list or reminder. That gives your brain the space to shut down and sleep deeply.

5. Increase your daily activity

Studies show that regular physical activity supports deeper sleep by helping regulate your circadian rhythm and reducing stress. Exercise also triggers physiological changes that make it easier for your body to relax later, such as lowering blood pressure.

That said, very intense workouts late in the evening can be too stimulating for some people, so earlier in the day is usually better if deep sleep is your goal.

a photo of a woman running outside

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Becca Caddy
Becca Caddy

Becca has written about homes, lifestyle and tech for brands such as TechRadar, T3, The Guardian and many more. Her first book, Screen Time, came out in January 2021 with Bonnier Books, within which she also explores how screen use affects sleep. For Tom's Guide Becca writes about sleep health and techniques, with the aim of helping readers to sleep better each night.

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