The Truth About Core Sleep (And The Four Sleep Stages)
If you’ve ever used a sleep tracker—whether it’s a Fitbit, Whoop, Apple Watch, or Oura—you’ve probably wondered, what is core sleep? It sounds important, right? Like it’s the foundation of your rest, something crucial to your health and well-being?
If it feels like a new term, that's because it is:
But Why?
According to Google trends, the search interest—meaning the frequency “core sleep” has been Googled over time—going all the way back to 2010, exploded overnight. There’s almost nothing and then in September 2022, boom! It becomes a thing.
Just a little more Googling reveals the emergence of “core sleep” just so happens to coincide with the Apple’s release of iOS 16, in reference to their watch.
What this article covers:
What is core sleep and why was the term invented
Is core sleep valid?
What the real sleep stages are
Proven ways to improve sleep
Here’s the Catch: "Core Sleep" is Not a Real Medical Term
Many people were coming to me and asking about their core sleep and initially, I wasn't sure how to answer, but after many instances and doing a deep dive, I’ll break it down for you here.
Different brands define it differently, but generally, core sleep is a way of bundling together certain sleep stages:
Fitbit: Defines core sleep as the combined time spent in light sleep (N2) and deep sleep (N3).
Whoop & Oura: Sometimes include REM sleep, but this varies based on their algorithms and software updates.
The problem? It’s a marketing term created by wearable companies and the definitions are arbitrary. The American Academy of Sleep Medicine (AASM) recognizes four specific sleep stages that we get into below—core sleep is not one of them.
Why the Term "Core Sleep" Exists
So why do these companies use a made up term? I have my suspicions (and I don’t want to get sued so I’m framing these as questions) but here are some possible reasons:
Device Limitations & Legal Liability? – Most wearables aren’t medical grade. They vaguely estimate sleep stages based on heart rate variability and movement. They do not use EEG brain wave readings (which are the gold standard for measuring sleep stages). Given the physical limitations of the devices themselves, their lawyers may have advised against using actual medical terms when the devices aren’t sensitive enough to detect the difference.
Consumer Simplicity? – Instead of explaining the complexities of sleep cycles, they package it as "core" vs. "REM" to make it easier for users to understand.
Brand Differentiation? – It’s a way for companies to create proprietary sleep scoring systems and keep users engaged in their ecosystem.
“How you feel is more important than what your sleep tracker says.”
Is "Core Sleep" Scientifically Valid?
Not really. The categorization is arbitrary and varies between brands. If you’re someone who prefers hearing it straight vs through a marketing filter, here’s what I would want to know:
Wearable sleep data is an estimate, not a diagnosis. Studies show they can be off by 20-30% compared to polysomnography (the gold standard sleep study).
Deep sleep (N3) and REM sleep are both crucial for recovery, memory, and overall health. The way these devices lump or separate them is more about software decisions or device limitations than actual sleep science. Any sleep doctor would want to get more into these details when doing an actual diagnosis.
If you’re improving your sleep based on wearable feedback, that’s great—but don’t take their metrics as gospel. When diagnosing problems, we use specific devices designed to generate readings we can trust well enough to order prescriptions.
Get Your Sleep Checkup
If you’re wondering what your sleep data might be trying to tell you, even if it’s from a wearable, we can take a peak.
Understanding Sleep Stages (And What AASM Recommends)
If you want to track your sleep in a way that's actually backed by science, it helps to understand the four sleep stages recognized by the American Academy of Sleep Medicine (AASM):
NREM Stage 1 (N1) – Light Sleep
The transition between wakefulness and sleep. Nodding off. The feeling of letting go.
Lasts about 5% of total sleep time in adults.
NREM Stage 2 (N2) – Light Sleep
A deeper stage of light sleep where heart rate slows and body temperature drops.
Makes up 45-55% of total sleep.
NREM Stage 3 (N3) – Deep Sleep (Slow-Wave Sleep)
The most restorative stage of sleep, crucial for physical recovery, immune function, and memory consolidation.
Typically 15-25% of total sleep.
REM Sleep – Dreaming and Cognitive Processing
Critical for emotional regulation, learning, and memory.
Typically 20-25% of total sleep, increasing in duration in the latter half of the night.
A healthy sleep cycle includes cycling through these stages multiple times per night, with deep sleep more common in the first half and REM sleep dominating the later half.
Final Take: Should You Pay Attention to "Core Sleep"?
If tracking sleep helps you make better lifestyle choices—like going to bed earlier or reducing caffeine—then go for it. But if you feel like you might have gotten a bit too fixated on hitting an arbitrary "core sleep" goal (that you now know was made up) it’s worth taking a step back.
Instead, focus on proven sleep hygiene habits:
Keep a consistent sleep schedule (even on weekends)
Limit screen time at least 30-60 minutes before bed
Optimize your bedroom for sleep (cool, dark, and quiet)
Avoid caffeine 6 hours before bedtime
Cut back on alcohol since it worsens sleep quality
If you snore or feel unrefreshed despite 7-9 hours of sleep, get screened for obstructive sleep apnea
At the end of the day, how you feel is more important than what your sleep tracker says.
What Do You Think?
Have you noticed wearables shaping how you think about sleep? Do you feel like "core sleep" metrics help or just add to stress?
And if you’re serious about improving your sleep, check out WakeWell’s personalized sleep solutions. Because real sleep science broken down by your doctor who knows you is going to beat mass consumer tech and marketing buzzwords every time.
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