Wake up, check your wearable, see a sleep score. For most people, that's where the interaction ends — a number with no clear sense of what fed into it or what to do if it's low three nights in a row. This guide breaks down what a sleep score actually measures, what "good" deep sleep and REM sleep look like, and what genuinely moves the number, versus what's mostly noise.
What a sleep score is actually built from
Most sleep scores — including the sleep component of the ONU Score — are built from a combination of:
- Total sleep duration relative to your typical need
- Sleep efficiency — time in bed actually spent asleep
- Time in each sleep stage, particularly deep sleep and REM
- Consistency — how closely your sleep and wake times match your usual pattern
- Restfulness signals, such as how many times you woke or stirred overnight
Because different wearables weigh these factors differently, the same night of sleep can produce different scores on different devices — which is exactly why the trend in your own score matters more than comparing your number to someone else's.
Sleep stages, in plain language
- Light sleep (NREM stages 1–2) — the transition into and largest share of total sleep.
- Deep sleep (NREM stage 3) — associated with physical restoration, typically roughly 13–23% of total sleep, concentrated earlier in the night.
- REM sleep — most associated with dreaming and memory processing, typically roughly 20–25% of total sleep, concentrated later in the night.
Because deep sleep clusters earlier and REM clusters later, cutting a night short at either end costs you a disproportionate amount of one stage — going to bed late costs deep sleep, waking up early costs REM.
What actually improves deep sleep
- Keep a consistent sleep and wake schedule, including on weekends.
- Avoid alcohol close to bedtime — commonly associated with reduced deep sleep.
- Exercise earlier in the day — associated with increased deep sleep the following night.
- Keep your bedroom cool — a slight core temperature drop helps initiate deeper stages.
What actually improves REM sleep
- Protect your full sleep window, especially the wake-up end.
- Manage evening alcohol and late caffeine — both linked to REM suppression.
- Keep stress in check before bed — see how stress physiology affects sleep architecture.
Why a low sleep score doesn't always mean a "bad" night
Sleep quality is one input into how tired you feel, but it isn't the only one — stress, hydration, and blood sugar patterns can produce fatigue independent of how you actually slept. If you consistently wake up tired despite a reasonable sleep score, ONU's guide to persistent fatigue looks at the other side of that question.
How ONU uses your sleep score
ONU reads sleep data directly from Apple Health, Whoop, Garmin, or the ONU Bracelet, and folds it into your daily ONU Score alongside recovery, activity, and stress signals.
When poor sleep quality is worth a medical conversation
It's worth raising with a doctor if you experience loud snoring or gasping during sleep, persistent difficulty falling or staying asleep over several weeks, or daytime sleepiness severe enough to affect daily functioning.
Frequently asked questions
Every claim, built on the same foundation
Plain-language, evidence-checked explanations of the biology behind your health data — organized by the kind of question that usually brings people here in the first place.
1Blood Test Results Explained
Cholesterol, non-HDL, A1C, and metabolic panels — decoded in plain English, including what a borderline number is actually worth flagging to your doctor.
2Sleep Score Explained
Deep sleep, REM, and why your score moves the way it does — plus research-backed ways to actually improve both, not just chase a higher number.
3Why Am I Always Tired?
Tired but can't sleep, dragging through mornings, waking up unrefreshed — the most common, evidence-backed explanations, in plain language.
4HRV Explained
What heart rate variability measures, what counts as a good resting heart rate, and how HRV connects to zone 2 training and real recovery.
5Cortisol and Stress
What cortisol actually does in your body, what the viral terms get right and wrong, and which foods and supplements genuinely affect stress.
6Acute vs. Chronic Stress
The actual difference between short-term stress and chronic burnout, and whether stress genuinely speeds up aging — without the scare tactics.
7Biological Age Testing
VO2 max, biological age calculators, and the longevity metrics everyone's suddenly discussing — what's evidence-backed, and what's still unproven.
8Full Body Scans & Preventive Checkups
What full-body MRI and CT scans actually involve, what physicians say about screening healthy adults, and where routine tracking fits in between.
9Magnesium & Ashwagandha: A Practical Guide
Which magnesium form actually helps sleep, when to take it, and how much ashwagandha is typically used in studies — no supplement-aisle guesswork.
How this library is built
Every article here goes through the same process: a claim isn't published until it's checked against a primary, authoritative source — the CDC, NIH, WHO, American Diabetes Association, American Heart Association, or peer-reviewed research, depending on the topic — and every article stays inside ONU's wellness-lane guardrail: educational and non-diagnostic, with no disease, treatment, or cure claims anywhere. Where the science is genuinely unsettled or still developing, these guides say so directly, rather than overstating certainty for the sake of a cleaner narrative.
Nothing in this library exists purely as content marketing disconnected from the product. Each article maps directly to something ONU's app actually measures and interprets: the HRV guide explains the exact metric behind your recovery score, the Blood Test Results guide explains the same interpretation logic your uploaded labs go through, and the Cortisol guide explains the physiology behind your daily stress signal. Reading a guide here should make your own ONU Score meaningfully easier to understand, not just deliver abstract information disconnected from your actual data.
If you want a broad orientation rather than jumping straight to one specific question, a reasonable starting sequence is: Blood Test Results Explained (the foundation for understanding any lab work), Sleep Score Explained and HRV Explained (the two most common wearable metrics people misunderstand), then Cortisol and Stress (the physiology behind the terminology dominating health conversations online right now). From there, the remaining guides go deeper into specific, related questions.
See how ONU turns this knowledge into your daily ONU Score in the full product overview.
ACTION
Know it before it happens
ONU provides you with tools to build your healthy future; now it's your time to use them!