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What Is Core Sleep? Unpacking The Misunderstood Sleep Metric

Julia Guerra
Author: Expert reviewer:
February 29, 2024
Julia Guerra
Health Writer
By Julia Guerra
Health Writer
Julia Guerra is a health and wellness writer reporting for mindbodygreen, Elite Daily, and INSIDER.
Shelby Harris, PsyD, DBSM
Expert review by
Shelby Harris, PsyD, DBSM
Clinical psychologist & Sleep specialist
Shelby Harris, PsyD, DBSM is a clinical psychologist and sleep specialist in private practice in NY. She is board certified in Behavioral Sleep Medicine and treats a wide variety of sleep, anxiety and depression issues using evidence-based, non-medication treatments. Before going into private practice, she was the longstanding director of the Behavioral Sleep Medicine Program at the Sleep-Wake Disorders Center at Montefiore Medical Center. She is a Clinical Associate Professor of Neurology and Psychiatry at the Albert Einstein College of Medicine in NYC.
February 29, 2024
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If you use a sleep tracker, there's a chance you've seen "core sleep" show up in your morning report. The thing is, core sleep isn't an officially recognized sleep stage, which makes figuring out what to do with your reading a little confusing.

We pulled back the covers on core sleep's various definitions, how it compares to sleep stages like REM, and what your core sleep reading says about your health.

The need-to-knows:

  • Core sleep is not an officially recognized sleep stage. The term "core sleep" is commonly misdefined as its own sleep stage. Most specialists use it as an umbrella term to describe sleep—and all of its stages.
  • Some specialists use "core sleep" to describe a grouping of sleep stages. "Core sleep" is sometimes used in reference to REM and the deep stages of slow-wave sleep, while Apple sleep trackers use it to umbrella the lightest sleep stages.

The stages of sleep 

Before we can talk about core sleep, we need to cover the four stages of sleep that are recognized by sleep experts—each with its own specific purpose:

  • Non-REM Stage 1: Sleep doctor Sarah Silverman PsyD, tells mindbodygreen that the stages begin with non-REM Stage 1, the lightest stage of sleep. Silverman calls this the "transitional stage" as it takes you from wakefulness to sleep by slowing your breathing and relaxing your muscles.
  • NREM Stage 2: Also a light stage of sleep, Silverman says NREM Stage 2 is where you spend most of the night. Your heart rate slows, your core body temperature drops, and your memories consolidate during this stage. 
  • NREM Stage 3 (also called deep sleep or slow-wave sleep): The third sleep stage is NREM Stage 3, also known as deep sleep. This is when your brain waves and vital functions, like your breath and heart rate, slow significantly. Deep sleep is essential for processes like tissue repair, muscle growth, hormone regulation, the clearing of brain waste via the glymphatic system, notes Silverman.
  • REM: REM sleep, or dream sleep1, is the last sleep stage, during which cognitive processing and emotion regulation ensue. "During REM, you are more likely to experience vivid dreams (although you can also dream in the other stages)," Silverman says. "There is usually an increase in heart rate and temperature while at the same time, your body becomes paralyzed to prevent the acting out of dreams."

Approximating the time your body spends in each stage via a sleep tracker like the Apple Watch, Fitbit, or Oura Ring can help give you a better sense of your overall sleep quality.

Where does "core sleep" come in?

Sleep specialists generally use the term "core sleep" as a reference to the minimum amount of sleep needed to function throughout the day, Silverman explains.

But some older research studies2 have also used "core sleep" to reference the deep sleep stages of REM and NREM Stage 3 sleep.

And here's where it gets even more confusing: Apple defines core sleep as Stages NREM Stage 1 and NREM Stage 2 sleep in its tracking. Fitbit and the Oura Ring, on the other hand, do not reference core sleep in their tracking.

How much core sleep do you need?

How much core sleep you need every night depends on a few key factors, such as your age, health, lifestyle, etc. It also depends on which definition of core sleep you’re adhering to.

Sleep needs are largely personal, but the Centers for Disease Control and Prevention (CDC)3 recommends adults between the ages of 18 and 60 get seven or more hours of total sleep per night for optimal functionality.

Silverman says between 45-50% of this time should be spent in deeper sleep stages, and the rest of your night can be in those lighter NREM1 and NREM2 stages.

It’s worth noting, however, that your sleep architecture might change as you age. Silverman reminds us that, compared to kids and teenagers, adults spend less time in deeper sleep stages and even less time after age 65.

"Sleep becomes lighter overall as a byproduct of age, medical history, and lifestyle factors such as light exposure, meal timing, substances (e.g., alcohol, nicotine, caffeine), activity level, stress level, etc.," Silverman says. 

8 reasons you’re low in core sleep

Each and every sleep stage is essential for health—even the lighter ones that may fall under the "core sleep" umbrella depending on who you ask. Here are some reasons you may be struggling to clock enough time asleep every night:

  • You have insomnia: If a person suffers from insomnia and has trouble initiating or maintaining sleep it may keep them out of certain sleep stages, Stuart Peirson, PhD, a sleep and circadian neuroscience researcher, explains.
  • You have sleep apnea: People who suffer from sleep apnea stop breathing for intervals of time during their sleep cycles. It’s a serious condition that affects a person’s overall health and sleep quality, with the Mayo Clinic reporting sleep apnea leads to "repeated awakenings" and makes restorative sleep nearly impossible to achieve. 
  • Your bedroom could use some work: Whether you’re defining core sleep as light sleep, deep sleep, or sleep in general, an uncomfortable sleep space negatively impacts it all.
  • Noise pollution: Are you constantly waking up to horns honking, a relentless neighborhood dog barking, or your roommates snoring? The American Public Health Association declared noise pollution a public health hazard in 2021, as outside noise disruptors can lead to a myriad of health problems, including poor sleep.
  • You drink alcohol or caffeine too close to bedtime: Alcohol can cause problems with sleep architecture, nervous system specialist and holistic nurse practitioner Victoria Albina, N.P., MPH previously told mindbodygreen. Since caffeine has such a long half-life, drinking it too close to bed can also mess with sleep.
  • You have too much blue light exposure: Whether you keep getting caught in late-night doom scrolls or like to read before bed on your phone or laptop, experts say blue light can have negative effects on your circadian rhythm4 (or sleep/wake cycle).

How to get more core sleep

Let’s say you’re diligently tracking your sleep, and your scores are lacking. Whether you're low in light sleep or REM and deep sleep, you want all of your stages to be well-represented to ensure you feel your best, mentally and physically. Here are a few foolproof ways to get more "core sleep":

1.

Transform your bedroom into a sleep oasis.

Your sleep space should be just that: a space dedicated to sleeping. Per the National Sleep Foundation, mastering sleep hygiene, which includes your sleep environment, is key to improving your overall sleep quality.

Set the mood by dimming the lights a half hour to an hour before you’d ideally like to be lights out, firing up your diffuser with a calming aroma, investing in a mattress and pillow that cater to your comfort preferences, spoiling yourself with quality bedding, etc.

2.

Get into a routine.

Another way to achieve A+ sleep hygiene is by setting and following a sleep schedule. If you’re going to bed at 9 p.m. one day and midnight the next, with the rest of the week in a similar pattern of disarray, your circadian rhythm is likely confused.

Get it in check by setting a realistic bedtime and waketime, and stick to it. Set your alarms and make sleep a priority. If making such a drastic shift doesn’t feel doable overnight, ease into your ideal routine by sleeping a little earlier each week or night until you’ve found a timeframe that works.

3.

Reduce your liquid intake before bed.

If you’re thirsty before bed, have a few sips of water to quench your thirst, but don’t down an entire glass (or two).

According to a study published in the British Journal of Nutrition5, the closer you drink before you sleep, the more likely you are to wake up in the middle of the night needing to use the bathroom. Its findings showed that drinking an hour or more before bed decreased a person’s chances of a mid-night pee break, so plan accordingly (if you can).

4.

Put your devices to bed, too. 

Using your phone too close to bed keeps your brain awake, thanks to the way blue light mimics daylight. It also puts you at risk of consuming content that can cause unnecessary stress.

A study published in the journal PLOS ONE6, in which participants had restricted phone access before bed for four weeks, found an increase in sleep duration and quality, among other benefits.

The "sleep doctor" Michael Breus, Ph.D., previously told mindbodygreen that all devices (smartphones, tablets, desktops, laptops, and, yes, even television) should ideally be shut off at least one hour before bedtime. 

5.

Don't eat dinner or drink alcohol too close to bedtime.

Ideally, you'll finish eating your last meal around three hours before bed, and sip your last alcoholic beverage at least four to six hours before bedtime to avoid sleep disruptions.

6.

Get your bedroom in sleep shape.

Making sure your thermostat reads around 68 degrees Fahrenheit or cooler, swapping regular curtains for blackout options, or investing in a new mattress can all help set up your space for sleep. Products such as sound machines and noise-canceling headphones can also drown out unwanted sound and improve your overall sleep quality.

7.

Manage your stress as much as possible.

Stress affects everything, so it’s no surprise that it can be your circadian rhythm’s kryptonite. If you’re feeling overly stressed and think it might be negatively affecting your sleep, try these remedies for dealing with it.

Other perspectives on tracking core sleep

While it can be beneficial to track your sleep (after all, knowledge is power, especially when it comes to your health), not all sleep experts recommend it.

Silverman, for example, says that, as an insomnia specialist, she doesn't suggest sleep trackers for her patients. "Sleep wearables aren’t able to differentiate between sleep and wake very well in individuals with insomnia," she notes.

People who are vulnerable to becoming unhealthily obsessed with the numbers may also want to avoid data tracking.

"Sleep tracking and focusing too much on the data can often lead to more worry about sleep, as I tend to see a lot of individuals who develop 'orthosomnia,' or an unhealthy obsession with achieving perfect sleep due to becoming fixated on the sleep stages," Silverman says.

"Most commonly, it is a fixation on whether there was enough REM or deep sleep during the night," Silverman continues, adding that this behavior can quickly become counterproductive. "It increases worry about sleep and tends to lead to a lack of self-trust or confidence in your ability to sleep well."

The accuracy of these trackers’ data is also cause for concern. According to Peirson, the only truly reliable way to measure your time in each sleep stage is polysomnography, or a sleep test performed in a lab that, measures "your brain waves, the oxygen level in your blood, and your heart rate and breathing during sleep."

If you are seriously concerned about your quality of sleep and/or have already noticed signs of a sleep disorder, Peirson strongly suggests seeking advice from a health care provider. They can help advise on whether a sleep study is necessary and appropriate for you.

The mindbodygreen POV

There is no universal definition for "core sleep." Some sleep experts use it as shorthand for the amount of sleep someone needs each night to function the next day (it's a little different for all of us). Some sleep trackers measure core sleep as the amount of time you spend in lighter sleep stages (NREM1 and NREM2).

No matter which definition you follow, enhancing your core sleep likely involves cleaning up your overall sleep routine so you spend an appropriate amount of time in each sleep stage.

You can do this by using less tech before bed, following a steady bedtime and wake-up time, watching what you eat and drink at night, or following a number of other sleep-promoting tips.

You can measure how you're tracking towards your goals with a wearable sleep tracker. Just remember that they're not 100% accurate and stressing over the numbers too much could do more harm than good. Simply clocking how you feel when you wake up and go about your day will also give you an idea of your sleep quality.

—Emma Loewe, mindbodygreen health & sustainability director

Frequently Asked Questions

How long should you be in core sleep?

How many hours of sleep a person needs to function most optimally throughout the day depends on a variety of factors, including their age, health status, and lifestyle. The CDC recommends adults clock in a minimum of seven hours of sleep per night, but this varies depending on the individual’s needs.

What is better for you core sleep or deep sleep?

Core sleep is not a sleep stage in and of itself. Depending on the expert, core sleep could mean light sleep, deep sleep, or all sleep stages. No matter the definition, all sleep stages are important to your sleep health, and no one stage is more important than another.

How is core sleep different than REM and deep sleep?

By Apple’s definition, "core sleep" encompasses the two lightest sleep stages, NREM Stage 1 and NREM Stage 2, so it's a precursor to the deeper sleep stages of REM sleep and deep sleep. However, most sleep experts define core sleep as the amount of sleep you need to function during the day, so it encompasses REM sleep and deep sleep.

The takeaway

There is no one clear definition of what core sleep is. It can refer to the light sleep stages, deep sleep stages, or the minimum amount of hours of sleep an individual needs to function properly, depending on who you talk to. But one thing all sleep experts agree on is that quality sleep is the most important sleep measurement. The goal is to wake up feeling good and ready to take on the day. If you’re waking up feeling groggy and a little off-kilter, try these tips for feeling more energized in the mornings.

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