TL;DR: Sleep duration 7 to 9 hours, sleep efficiency above 85 percent, sleep latency 10 to 20 minutes, WASO under 30 minutes, deep sleep above 60 minutes, REM above 90 minutes, SpO2 above 95 percent. Those are the robust metrics. Sleep scores and single-night stage percentages are mostly marketing. Trends over 2 weeks beat any individual night.
This article is for information only and replaces neither medical advice nor a sleep physician. If you suspect sleep apnea or chronic insomnia, the clarification belongs with a specialist. Intake, dose and therapy of supplements or medicines should be clarified with a doctor; this article makes no recommendation on their use.
Why Track Sleep at All
Your subjective feeling on waking is a poor indicator of sleep quality. Most people underestimate deep sleep deficits and overestimate how long they actually slept. A wearable delivers objective data: when did you fall asleep, how often were you awake, how stable was your heart rate? Those are numbers you would never see without measurement.
The real value is not in a single night. It is in weekly and monthly trends. You see your deep sleep drop by 40 minutes after three nights of beer. You notice your HRV takes three days to recover from long-haul travel. And you catch early signals when something changes — a rising resting heart rate over two weeks is often the first sign of an infection or an overtraining phase.
Combine sleep tracking with biomarker data and you see patterns that are invisible in isolation. Low ferritin and poor sleep? High morning cortisol and early waking? Those are the patterns that matter.
The Four Sleep Stages in Brief
A sleep cycle lasts 90 to 120 minutes. You run through 4 to 6 cycles per night. Each cycle contains four stages with distinct functions.
| Stage | Share | Function |
|---|---|---|
| N1 (falling asleep) | 2 to 5 % | Transition from wake |
| N2 (light sleep) | 45 to 55 % | Memory consolidation, muscle relaxation |
| N3 (deep sleep, SWS) | 13 to 23 % | Physical recovery, growth hormone peak |
| REM | 20 to 25 % | Cognitive consolidation, learning, emotion processing |
Deep sleep (N3) dominates the first half of the night. Going to bed early gives you more of it. Growth hormone release, immune repair and muscle recovery run here.
REM dominates the second half of the night. If you sleep only 6 hours you disproportionately lose REM. You feel it the next day as reduced cognition and worse emotional regulation.
A concrete example: you go to bed at 11 PM and get up at 6 AM — that is 7 hours in bed. At 85 percent efficiency that is about 6 hours of sleep split across 4 to 5 cycles. If you go to bed at 1 AM and get up at 7 AM, you miss an entire cycle with full deep sleep.
The Core Metrics That Matter
These eight numbers are the foundation. Everything else is decoration.
Sleep Duration
Adults need 7 to 9 hours. Under 6 hours raises the risk of cardiovascular disease, insulin resistance and cognitive decline. Regularly over 9 hours suggests poor sleep quality or an underlying condition. Find your personal optimum over 4 weeks of tracking — if you thrive on 7.5 hours, you do not need 9. How widespread sleep deprivation is in Germany, Austria and Switzerland is shown in our sleep deprivation statistics for the DACH region.
Sleep Efficiency
Sleep efficiency = actual sleep time / time in bed. Good: above 85 percent. Excellent: above 90 percent. If you spend 8 hours in bed but sleep only 6.5, your efficiency is 81 percent — time is being wasted. Typical causes: too much WASO, long sleep latency, going to bed too early.
Sleep Latency
The time from lights-out to falling asleep. Ideal: 10 to 20 minutes. Under 5 minutes usually means you are overtired. Above 30 minutes points to sleep-onset problems — typical causes are afternoon caffeine, too much blue light before bed, or nighttime rumination.
WASO (Wake After Sleep Onset)
Time you spent awake after falling asleep. Ideal: under 30 minutes. Brief wake-ups are normal and usually unnoticed. WASO above 60 minutes fragments sleep and makes it unrefreshing. Common causes: alcohol, warm bedroom, noise, full stomach.
Deep Sleep
Target: above 60 minutes per night. Individual wearable numbers are noisy, but your personal 2-week average is reliable. Alcohol measurably cuts deep sleep even if you fall asleep faster. Late exercise (after 8 PM) and warm bedroom temperature also reduce deep sleep.
REM Sleep
Target: above 90 minutes per night. REM concentrates in the second half of the night — early risers lose disproportionate REM. Alcohol is the strongest REM killer: two glasses of wine can cut REM by 20 to 30 percent. Antidepressants, THC and late meals have similar effects.
Breathing Rate
Healthy adults breathe 12 to 20 times per minute during sleep. Your personal baseline matters more than the normal range. A rise of 2 to 3 breaths per minute across several nights is often an early warning sign of infection — before you notice symptoms.
Nighttime Heart Rate and HRV
Nighttime heart rate typically sits 5 to 10 bpm below your daytime resting heart rate. HRV is higher at night than during the day and shows how well your parasympathetic nervous system is recovering. Read them together: falling heart rate plus rising HRV equals good recovery. More on interpretation in the wearable data quality guide.
Body Temperature and SpO2
Core body temperature drops about 0.5 degrees Celsius overnight — that is a sleep signal. A bedroom that is too warm blocks the drop and degrades deep sleep. SpO2 (blood oxygen) should stay above 95 percent. Repeated drops below 92 percent are a signal to get sleep apnea checked.
Overrated Metrics
Three numbers are heavily marketed by wearable brands but rarely help decisions.
Sleep Score. A single value between 0 and 100 that mixes several raw data points. Every manufacturer uses its own formula. The score is good for motivation but poor for root-cause analysis. When your score drops from 85 to 68 you do not know why — you still have to check the individual values.
Single-night deep or REM percentages. Wearable stage classification diverges markedly from polysomnography in validation studies. Individual nights are noise. Only weekly trends become reliable.
Detailed dream-cycle views from wrist wearables. No wearable outside a sleep lab can precisely detect individual REM episodes or dream events. The polished charts with exact stage transitions are algorithmic interpolations, not measurements.
Device Comparison: Which Wearable for Whom
The right device depends on what you want to optimize.
| Device | Strengths | Weaknesses |
|---|---|---|
| Polysomnography (sleep lab) | Gold standard, EEG-based | One-time, expensive, only on suspected diagnosis |
| Oura Ring | Well validated for sleep vs. wake, small form factor | Imprecise stage assignment, battery 5 to 7 days, subscription model |
| Whoop | Strong HRV and heart rate, recovery score | Limited stage detection, subscription required |
| Apple Watch / Garmin | Good trends, many extras | Coarser stage detection, must wear at night |
| Withings Sleep Analyzer | Mattress sensor, no wear required, apnea detection | Home only, one sensor per bed |
The de Zambotti (2019) Oura-PSG study remains the most-cited reference: 96 percent sensitivity for detecting sleep, but markedly weaker specificity for wake and markedly weaker agreement for stage classification. That means duration and efficiency are comparatively accurate, while individual stages are error-prone.
If you already have a wearable, do not hop between devices. Each one has its own baseline and cross-brand comparisons are worthless.
How to Actually Use the Data
Most wearable users stare at single nights and draw wrong conclusions. Four rules make the difference.
Establish a 2-week baseline. Before testing any intervention you need averages for duration, efficiency, deep sleep, REM, HRV and resting heart rate. That baseline is your reference — not the average of other users.
Weekly trends, not individual nights. One bad night is normal. Three in a row is a signal. Always look at the 7-day rolling mean. It filters noise and shows real change.
Correlate with subjective feel. Log a 1 to 10 score every day for your perceived recovery. Compare it with wearable data. If the two diverge, either the sensor is off or you are ignoring a signal from your body.
Observe changes one at a time (N=1). It makes sense to change only one variable at a time. 2 weeks without alcohol. 2 weeks with a cooler bedroom. 2 weeks with fixed bedtimes. If several things change at once, it stays unclear what made the difference. The full methodology is in the insight-sprint playbook.
Spotting Common Sleep Problems in the Data
Your wearable can reveal patterns that point to concrete issues. Three examples with clear signatures.
Sleep apnea. Repeated SpO2 drops below 92 percent, elevated heart rate spikes at night, fragmented sleep despite adequate time in bed. If you wake up exhausted after 8 hours in bed, snore loudly and your partner notices breathing pauses, book a sleep lab. Apnea is treatable, but untreated it is a risk factor for heart attack and diabetes.
Alcohol effect. Two glasses of wine cut REM by 20 to 30 percent, raise WASO by 20 to 40 minutes and measurably drop HRV. Typical pattern: you fall asleep faster (short latency) but wake up between 3 and 5 AM and lose REM in the second half of the night. The effect is clearly visible in wearables and lasts 24 to 48 hours.
Cortisol dysregulation. Early waking between 3 and 5 AM, elevated heart rate in that window, inability to fall back asleep. The pattern shows up with chronic stress, late evening meals or elevated evening cortisol. If it persists for weeks, a salivary cortisol day profile is worth running.
Factors That Influence Sleep
Six factors with studies behind them. The order is not a recommendation for action; which measures suit your individual case is something to clarify with your doctor if needed.
1. Light exposure. Studies examine the influence of morning light (10 to 30 minutes, outside, no window in between, within an hour of waking) and of less blue light in the 2 hours before bed on the circadian rhythm. Associations with sleep latency and deep sleep are reported.
2. Bedroom temperature. Core body temperature drops about 0.5 degrees Celsius overnight, which is considered favorable for deep sleep. A warm bedroom (above 20 degrees) can block that drop; 16 to 19 degrees is considered favorable. The influence on deep sleep is measurable in wearables.
3. Caffeine. Caffeine has a half-life of 5 to 7 hours. An espresso at 3 PM is still about 25 percent in the body at 11 PM and can measurably reduce deep sleep. Sensitivity differs greatly between individuals.
4. Alcohol. Alcohol is the most clearly visible influence on REM. Even two glasses of wine cut REM by 20 to 30 percent and noticeably raise WASO. The effect is usually visible in a wearable within a week.
5. Consistent timing. Steady bed and wake times (about ±30 minutes), even on weekends, are associated with more stable sleep. Shifts larger than 90 minutes act like a small jet lag and take 2 to 3 days to recover. The cyclic routine playbook describes how consistency can be built.
6. Magnesium as a supplement. Magnesium supplements are being studied as a sleep aid; the evidence is limited and inconsistent, and a benefit is not established. Whether, in which form and in which amount a supplement is appropriate is a question for a doctor, especially with pre-existing conditions or regular medication. Background on the forms is in the magnesium forms guide.
Integrating Sleep Data Into Your Dashboard
Wearable tracking on its own only gets you so far. The next step is linking it with biomarkers and your own notes on nutrition and supplements. Your sleep metrics stay in your wearable’s app; lab values and supplement log live in Lab2go. Put the two side by side yourself and you spot patterns you would never find in isolation. Interpreting the values belongs in a doctor’s hands.
Correlations worth watching:
- Low ferritin correlates with restless-leg symptoms and fragmented sleep
- Vitamin D deficiency often coincides with poor sleep quality
- High evening cortisol shows up as early waking between 3 and 5 AM
- Low HRV over multiple weeks correlates with elevated inflammation markers
The available plans differ in how many biomarkers and profiles you keep. You keep your full history on Free and Pro alike. To get started with biomarker trends and a supplement log, the Free plan is enough; Pro removes the limits.
Conclusion: Trends Beat Single Nights
Sleep tracking is not an end in itself. It is a tool to relate habits to sleep data: how strong is the alcohol effect? Is my new training too intense? You answer those questions with weekly trends, not single nights. Health conclusions belong in a doctor’s hands.
Three steps to start:
- Set a baseline. Track 14 nights without changing anything. Note averages for duration, efficiency, deep sleep, REM and HRV.
- Observe one change. Bedroom temperature, bedtimes or alcohol abstinence. 2 weeks consistently.
- Compare weekly values. Not night by night — week 1 vs. week 2 vs. week 3.
To go deeper, combine sleep tracking with long-term biomarker tracking and use the insight-sprint method to test interventions cleanly. Check the features and pricing if you want both on one platform.
This article is for information only and does not replace medical advice. If you suspect sleep apnea, chronic insomnia or daytime fatigue despite enough time in bed, see a doctor or sleep physician. Intake, dose and therapy should be clarified with a doctor.
Article FAQ
- Which sleep metric matters most?
- Sleep duration and sleep efficiency are the most robust markers. 7 to 9 hours in bed, with at least 85 percent of that actually sleeping, is the baseline. Individual deep sleep or REM values from wrist-worn wearables are unreliable — in validation studies, stage classification diverges markedly from polysomnography. They become meaningful only when you look at consistent weekly trends.
- How much deep sleep per night is normal?
- Deep sleep (N3, slow-wave sleep) makes up 13 to 23 percent of sleep time, roughly 60 to 110 minutes for 7 to 8 hours of total sleep. It declines with age. What matters is your personal trend, not comparisons with others. Alcohol, late exercise and warm bedroom temperature all measurably reduce deep sleep.
- How accurate is my wearable at detecting sleep stages?
- No consumer wearable matches the accuracy of polysomnography. Oura, Whoop and Apple Watch are usable for telling sleep from wake, but for assigning individual sleep stages they diverge markedly in validation studies. Individual stage percentages are therefore error-prone, while trends over 2 to 4 weeks are reliable. For medical diagnoses you always need a sleep lab.
- What does a Sleep Score actually mean?
- A Sleep Score is a marketing number that condenses several raw data points into a single value between 0 and 100. Every manufacturer uses its own formula. The score is good for motivation but poor for data-driven decisions. Look at the individual values instead: duration, efficiency, WASO, deep sleep, REM, HRV.
- Why do I wake up at night even without drinking anything?
- Brief wake-ups are normal. Healthy adults wake 4 to 6 times per night without noticing. WASO (Wake After Sleep Onset) under 30 minutes is ideal. Causes of higher WASO: alcohol in the last 6 hours, cortisol dysregulation (typically 3 to 5 AM), bedroom above 20 degrees Celsius, or a restless sleep partner.
- What does an SpO2 drop at night indicate?
- SpO2 (blood oxygen) should stay above 95 percent through the night. Repeated drops below 92 percent suggest sleep apnea, especially combined with snoring and daytime sleepiness. Oura, Apple Watch and Whoop measure SpO2 optically and accuracy is limited, but repeated patterns are a clear signal to book a sleep lab.
- What influences nighttime heart rate?
- Nighttime heart rate typically sits 5 to 10 bpm below your daytime resting heart rate. If it is higher, possible influences include alcohol, late meals, infection, stress, hard training or a warm bedroom. Persistently abnormal values should be clarified with a doctor.
- How long should I collect sleep data before drawing conclusions?
- At least 14 nights for a baseline before you assess changes to your habits. Single nights are too noisy. To compare a change (for example bedroom temperature or bedtime), use 2 weeks with and 2 weeks without it, then compare weekly averages. Single nights easily lead to wrong conclusions.
This article is for general information only and is not a substitute for individual medical advice, diagnosis, or treatment. Discuss any changes to your diet, supplementation, or medication with a qualified healthcare professional.
Maritta Schmid, Heilpraktikerin (licence under the German Heilpraktikergesetz; non-medical health practitioner), Licence under the German Heilpraktikergesetz, issued by Gesundheitsamt Heilbronn (February 2010), Supervisory authority: Landratsamt Ostalbkreis – Gesundheitsamt Aalen
Heilpraktikerin & Founder
Schwäbisch Gmünd, Germany
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