Apple Watch · HealthKit · iOS

Know how your body is ageing —
measured against you.

Your Apple Watch already records the signals that show how well you're ageing. Kreev turns them into a free Healthspan Index, a daily Recovery Score and a VO₂ Max trend — all measured against your own baseline, not a population average. Nothing extra to buy or wear, and your health data never leaves your iPhone.

Launching soon in the US and Canada

iPhone · iOS 16+ · Apple Watch Series 4+

Join the waitlist

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Coming to the App Store

Coming soon.

Ageing Well Is a Measurable Thing.

Kreev reads what your Apple Watch already records and turns it into a small number of measurements you can act on. It is not a training app, and you do not have to be an athlete to use it: the things it watches — heart-rate variability, resting heart rate, sleep, VO2 Max — are the same markers that track how well a body is ageing, whether you are chasing a personal best or simply intending to still be doing this in twenty years.

Two of those matter more than the rest. HRV is the clearest daily read on whether your nervous system is coping with the load you are putting on it. Cardiorespiratory fitness — VO2 Max — is strongly associated with living longer, and unlike most things on a lab report, you can move it.

Kreev measures both against you, not against a population. And because slow change is the kind nobody notices, the Boiling Frog detector compares your recent months to your earlier ones and tells you when something has drifted — a VO2 Max that has slipped three points since spring, a bedtime that has crept later every week. That is the decline that does damage, and it is invisible day to day.

Newest: your Healthspan Index — five ageing markers scored into one number out of 100, free — and Kreev on your Apple Watch, where today's numbers sit on your watch face and are already current before you pick up your phone.

$0 Hardware Uses your Apple Watch
Personal Baseline Compared to you, not averages
Open Algorithm We show our math
Privacy First Never leaves your iPhone
100% On-Device No account, no server, no AI
Healthspan Index

Your Healthspan Index.
Five Markers, One Number.

Five measurements that track how well you are ageing — VO2 Max, HRV, resting heart rate, sleep, and sleep regularity — each scored out of 20, for one number out of 100. Nothing is weighted by your age or compared to anyone else: every component is scored on your own data. The ring shows which of the five is carrying you and which is holding you back, and the whole breakdown is free.

  • VO₂ Max — how much oxygen your body can use at peak effort. Cardiorespiratory fitness is strongly associated with longevity.
  • HRV — how flexibly your nervous system responds; a daily read on how well you're coping with load.
  • Resting heart rate — your heart's efficiency at rest. A lower trend usually goes with better fitness.
  • Sleep — how much you sleep against your own baseline.
  • Sleep regularity — how consistent your bed and wake times are.

The Index and its breakdown are free. The six-month trend is Sentinel.

75–100
Exceptional Ageing markers in your strongest range
50–74
Solid Foundation A healthy, sustainable baseline
25–49
Building Phase Real room to strengthen your markers
0–24
Needs Attention Worth a closer look across the board
71
Solid Foundation
VO2 Max
HRV
Resting HR
Sleep
Sleep Regularity

Free The Index and its breakdown

Sentinel The six-month trend

The Recovery Score

A Score That
Doesn't Flatter You.

On Kreev's scale, 50 means your usual — not "half recovered." The scale is centred on your own 30-day baseline, about 15 points for each standard deviation of your own variation, so an ordinary morning lands near 50 and an 80 is genuinely exceptional. A score moves only when your body does.

Free A real morning: 52, right at baseline

Free the number · Sentinel the ranking

VO₂ Max

Your Aerobic Fitness,
Calculated by Kreev.

Kreev calculates VO₂ Max itself from your heart-rate data, using the published Heart Rate Ratio method (Uth, Sørensen, Overgaard & Pedersen, European Journal of Applied Physiology, 2004), Once one of your workouts includes a hard, sustained effort, it uses the highest heart rate you've recorded within the last 12 months, so the estimate follows you as you age. Until then it uses a formula based on your age and sex. The app labels each estimate “Personalized” or “Formula-based”. It is an estimate — a trend to watch, not a lab test.

Why it matters: cardiorespiratory fitness is strongly associated with lower all-cause mortality (Kodama et al., JAMA, 2009; Mandsager et al., JAMA Network Open, 2018).

Your VO₂ Max number is free. Where it ranks against ACSM norms for your age and sex — and how far you are from the next band — is Sentinel.

Sentinel · Boiling Frog detector

The Decline You'd
Never Notice.

Day to day, a slow drift looks like nothing: every reading is within your normal variation. Kreev compares your last 30 days with a 60-day reference window that ends where those 30 begin, and tells you when a metric has moved away from where it used to sit — a resting heart rate that has crept up, a VO₂ Max that has slipped.

It needs about 45 days of data inside the reference window. Because Kreev imports your last 30 days from Apple Health when you install, someone who already wears their watch typically sees it after roughly six weeks of use.

60-day reference window last 30 days today 90 days ago
Illustrative — not real data. The dashed lines are the average of each window.
Sentinel · Autonomic Archive & Your Patterns

Your Good Days and Hard Days — Without Keeping a Diary.

Kreev finds what shapes your recovery from data your watch already collects. There is nothing to log.

The Autonomic Archive

One heatmap per month for your last six months, each day coloured by its Recovery Score. Runs of amber or red days show accumulated load that a monthly average hides; a weekday that is always low shows up as a column.

Your Patterns

What the app has learned from your own history, in plain sentences. It looks at:

  • how your sleep affects next-day HRV;
  • how a hard day's strain affects next-day recovery;
  • how your step count affects next-day strain;
  • whether a raised breathing rate comes before a lower recovery;
  • which weekday runs lowest, and whether shorter sleep the night before comes first;
  • whether regular bedtimes go with better HRV;
  • and, once you use the daily check-in, how your own feeling tracks your scores.

Free shows your most frequent bottleneck and your strongest pattern. Sentinel adds the Archive, the weekday read and every pattern with its numbers.

Patterns need about 60 days of history — with the 30-day import, roughly a month of use — and the Archive fills in month by month. These are patterns in your measured data (sleep, strain, steps, breathing rate, heart rate, weekday), not causes.

Pricing

Free tells you where you stand today. Sentinel tells you where you're heading.

Your Healthspan Index, Recovery Score, Strain and VO₂ Max are free — no extra hardware. Sentinel adds the direction of travel and the full interpretation layer, plus the Kreev Apple Watch app, for $5.99/month or $39.99/year (~44% off). New subscribers get 1 month of Sentinel free through the App Store; unless you cancel before it ends, it converts to the plan you chose. Cancel anytime in iPhone Settings → Apple ID → Subscriptions.

The question Free Sentinel
How is my body today?Recovery Score, Strain, overnight metrics, Healthspan Index with its five-component breakdown, VO₂ Max valueEverything in Free
Where do I stand against people my age?—VO₂ Max ACSM ranking — category, percentile band, distance to the next band
Is it getting better or worse?7-day trends and the current monthSix-month Healthspan trend, extended history, Boiling Frog detector
What shapes my good and bad days?A preview: your most frequent recovery bottleneck and your strongest patternAutonomic Archive (six monthly heatmaps), the weekday read, every pattern with its numbers
Why, in plain English?—Daily narrative, Weekly Debrief, Month in Review, the “Why” behind each score
How should I train this week?StrainRecovery vs Strain, This Week suggested hard days, detailed drill-downs, Guardian Alerts
At a glance on my wrist?The Kreev brand-mark complicationWatch app + Recovery, Strain, VO₂ Max and Healthspan complications

Kreev's free tier is the iPhone app. The Kreev watch app and its Recovery, Strain, VO2 Max and Healthspan complications are part of Sentinel — you can still add one to a watch face for free, but it will show a lock instead of a reading.

Introductory offers are administered by Apple and available once per Apple ID. Your subscription follows your Apple ID, not your phone — use Settings → Restore Purchases on a new device. We remind you before each renewal. Nothing is deleted when a subscription ends: your history stays on your iPhone and the free tier keeps working.

Join the waitlist
The Recovery Score

One Number.
Your Entire Day.

Every morning, Kreev analyses your overnight biometrics — heart rate variability, resting heart rate, sleep quality, activity load, respiratory rate — and distills them into a single Recovery Score from 0 to 100. Not a guess. Not a population average. A precise reading of how your body recovered, compared to your own history.

65–100
Above Baseline Recovery above your personal norm
50–64
At Baseline Typical recovery for your body
35–49
Below Baseline Recovery below your personal norm
0–34
Recovery Needed Significant recovery deficit
BASELINE 35% 50% 65%
0%

vs your personal baseline

0 —

Recovery Compass

62ms HRV
52bpm RHR
7.8h Sleep
8,420 Steps
Your Personal Baseline

Your Score Compares You to You.

Most wellness apps compare you to population averages — norms built from studies that may not reflect your age, fitness level, or physiology. Kreev is different. It builds a rolling baseline from your own recent history and measures how today deviates from your normal. An elite athlete with a resting heart rate of 42 bpm and a weekend jogger at 72 bpm both get scores that reflect their individual recovery — because the reference point is always you.

01
Wear

Your Apple Watch collects heart rate, HRV, sleep, steps, respiratory rate, and VO2 Max throughout the day and night.

02
Baseline

Kreev builds a rolling average of your last 30 days. This is your personal normal — unique to your body.

03
Score

Each morning, today's metrics are compared to your baseline using Z-score statistics. The result is your Recovery Score.

Biometric Intelligence

Seven Signals. One Picture.

Every metric has a story. Kreev reads them all.

Healthspan Index — Free
Five measures that track how well you're ageing — VO2 Max, HRV, Resting Heart Rate, Sleep, and Sleep Regularity — scored out of 20 each, for one number out of 100. Free, with the full component breakdown.
0
Solid Foundation
Strain
A daily measure of cumulative physiological demand from exercise and activity. When strain is high relative to your baseline, Kreev flags it so you can balance load with recovery. Sentinel adds the Recovery vs Strain chart, which shows how the two interact over time.
0%
LOW
Resting Heart Rate
0 bpm
Your heart's resting efficiency. Lower trends signal improving cardiovascular fitness.
Heart Rate Variability
0 ms
The variation between heartbeats. Higher HRV correlates with better autonomic recovery.
Sleep Duration
0 hrs
Total sleep versus your baseline. Consistency indicates more than one good night.
Steps / Activity
0
Daily movement load. Kreev tracks whether activity is adequate — or pushing too hard.
Respiratory Rate
0 br/min
Breathing rate during sleep. Subtle changes can signal shifts in recovery status.
VO2 Max
0 ml/kg/min
Peak oxygen uptake — your aerobic ceiling. Tracked over months to measure fitness trajectory.

See it in the app

Recovery Insight On-device
64 Recovery Score
HRV 54ms ▼ −7%
Resting HR 56 bpm ▲ +2 bpm
Sleep 7.1h — Baseline
Deep Sleep 18% — Baseline
Activity Load 6,820 steps — Moderate

Your recovery score of 64 indicates you are tracking at your personal baseline today. HRV at 54 ms sits 7% below your 30-day average of 58 ms, pointing to mild residual autonomic load — likely from recent training sessions. Resting heart rate at 56 bpm is marginally elevated from your typical 54 bpm, consistent with moderate, unresolved physiological demand. Sleep duration of 7.1 hours and deep sleep at 18% of total time are in line with your recent norms. Yesterday's moderate activity load leaves no significant cumulative strain. You are recovered to your typical level — a standard training day is appropriate.

Sentinel · What your morning looks like

Your Score, Explained — On Your iPhone.

Every morning, Kreev writes a narrative that explains exactly why your Recovery Score is what it is: which metrics moved, how far from your baseline, and what pattern that fits. It is produced by a rules-based engine that runs on your phone — no AI service, no upload, the same inputs always produce the same explanation. No medical jargon. No vague advice. Just your data, interpreted.

Generated entirely on your iPhone — nothing is transmitted

Works offline and in airplane mode

The methodology is published in our Terms of Service

Sentinel A real morning's narrative

For people who train

Train With Your Recovery in View.

Everything above still applies. If you also train, Kreev adds load to the picture. The guide goes deeper →

Strain Free

A daily measure of the physiological demand you've put on your body, from your actual heart rate, shown against your own baseline.

Recovery vs Strain Sentinel

A side-by-side chart of how your recovery and your load interact over time.

This Week Sentinel

Suggests days for harder sessions from your recovery history, and where the easy days fall. It is not a prescription.

Heart-rate recovery Sentinel · Apple Watch

Post-session heart-rate recovery, measured against your own history rather than a population table.

Apple Watch

Your Numbers, On Your Wrist.

Part of Sentinel.

Kreev's watch app shows the same four numbers as your phone — Recovery, Strain, VO2 Max and your Healthspan Index — plus today's training context. Add a Kreev complication to any watch face and it is there at a glance, updated in the background before you pick up your phone.

Your iPhone does all the computing and sends only the finished scores to your watch, directly, over Apple's own encrypted connection. Kreev's watch app has no access to Apple Health at all — there is nothing on your wrist to leak, and nothing goes over the internet to get there.

Four metrics, one tap.

Recovery with your HRV and Resting HR movement, Strain, VO2 Max with its ACSM ranking, and your Healthspan Index — one scrolling screen, no menus.

Five complications.

Recovery, Strain, VO2 Max, Healthspan, and a Kreev brand mark for faces with only one slot. Most faces hold one to four, so mix and match.

How to add a Kreev complication to your watch face →

Current when you look.

Kreev refreshes in the background as Apple Health delivers new readings — so the morning number on your wrist is already today's, before you open anything.

See it on your wrist

Transparency

We Explain How Your Score Works.
No Black Boxes.

Kreev publishes its scoring methodology, names every company that touches your data, and lets you export or erase everything yourself. Because it runs on your iPhone, nobody receives your health data.

Algorithm Transparency

Your Recovery Score uses Z-score statistics against your personal rolling baseline. We explain the methodology in our Terms of Service — no black boxes.

No Demographic Profiling

Your score doesn't factor in age, sex, or ethnicity. Every user is compared to their own history — not a population average.

No Ads. No Tracking.

We don't use advertising cookies. We don't sell your data. We don't share it with data brokers. Ever.

Your Rights, Documented

CCPA/CPRA and MHMDA rights, PIPEDA compliance, and a published methodology. There is no account to delete — Settings → Reset This Device erases everything, and deleting the app does too. Export My Data gives you a full copy; Import My Data brings it back.

Read our Privacy Policy, Terms of Service, and Consumer Health Data Privacy Policy.

Frequently Asked Questions

Questions? We've Got Answers.

Your Recovery Score (0–100) reflects your body's readiness based on your personal baseline. It is computed from sleep duration, heart rate variability, resting heart rate, activity load, and respiratory rate — all compared to your own recent history using Z-score statistics. Scores above 75 indicate high readiness; below 50 suggests your body may benefit from more recovery time.
An iPhone running iOS 16 or later and an Apple Watch Series 4 or newer (watchOS 9+). Series 4 and above support all 6 biometric signals. Older Apple Watch models support basic metrics but lack respiratory rate and VO2 Max.
Kreev compares your metrics to your own personal baseline — not population averages. We publish our scoring methodology and name every company that touches your data. Plus, there's no hardware to buy and no 12-month commitment.
No. Kreev is a personal health informatics tool. It provides analytical observations about your biometric patterns — not medical advice, diagnoses, or treatment recommendations. Always consult a healthcare professional for medical concerns.
Strain (0–100) quantifies the cumulative physiological demand your body is under from exercise, sleep debt, and autonomic stress. LOW (0–34) — your body is managing stress well. MODERATE (35–64) — normal training stimulus. HIGH (65–100) — significant load; prioritize recovery before your next hard session.
Every morning, Kreev's rules-based engine — running entirely on your iPhone — reads your overnight metrics, compares each to your personal baseline, and composes a plain-language explanation from a fixed library of phrases. The same inputs always produce the same narrative, nothing is sent anywhere, and it works with your phone offline. No AI service is involved.
No. Your health data never leaves your iPhone — Kreev has no server to send it to. Kreev sends your scores to your own Apple Watch and to nobody else. Two services receive limited non-health information: RevenueCat, which manages your subscription (an anonymous identifier and your App Store receipt), and Aptabase, which counts anonymous feature usage with no identifiers and no health values. No advertisers, no data brokers, no AI providers. Every recipient is named in our Privacy Policy.
Yes, instantly and yourself — there is no account and no server copy. In the app, Settings → Reset This Device permanently erases every metric, score, narrative and check-in on your phone, and on your Apple Watch too. Deleting the app does the same. If you use iCloud Backup, a copy may remain in your own Apple backup until it is replaced; that copy is yours and Apple's, and Kreev cannot access it.
Free: Recovery Score, Strain, VO2 Max (the number, always), Healthspan Index with its full component breakdown, today's metrics (7-day trends), and the current month of history, plus a free preview of your History tab (your most frequent recovery bottleneck and strongest pattern). Sentinel adds the full narratives (daily, Weekly Debrief, Month in Review), the "Why" contributing-factors analysis, Recovery vs Strain, VO2 ACSM ranking, trends and history beyond the current month, detailed drill-downs, the six-month Healthspan Index trend, Autonomic Archive, Guardian Alerts, and the Kreev Apple Watch app.
Sentinel comes with a 1-month introductory free trial through the App Store. You choose a plan when you start it, Apple handles it, and it is available once per Apple ID. Unless you cancel at least 24 hours before it ends, it converts to the plan you chose. Nothing is deleted when a trial ends — your scores and history stay on your iPhone.
Because 50 means "at your normal," not "half recovered" — the scale is anchored to your own baseline, roughly ±15 points per standard deviation of your own variation. An ordinary morning scores around 50 by construction; an 80 is genuinely exceptional. See the scale.
Yes, unless you cancel. An App Store introductory trial converts to the paid plan you selected when it ends. Cancel anytime before then in iPhone Settings → Apple ID → Subscriptions and you will not be charged. We also remind you before every renewal.
Yes. Settings → Export My Data writes your full history — every metric, score, narrative and check-in — to a single JSON file and opens the share sheet so you can save it to Files, AirDrop it, or email it to yourself. Settings → Import My Data brings it back onto any iPhone.
Kreev keeps your history only on your iPhone, so Kreev itself cannot restore it. Two things protect it: iCloud Backup, which includes Kreev's data and restores it automatically on a new iPhone; and Export My Data, a file you keep yourself and can import on a new device. Your Sentinel subscription is separate — it follows your Apple ID (Settings → Restore Purchases).
We take privacy seriously. Read our Privacy Policy, Terms of Service, and Consumer Health Data Privacy Policy. We comply with CCPA/CPRA, MHMDA, and PIPEDA. You control automated scoring directly: revoke Apple Health access or delete the app, and the scores stop.
Contact us at support@kreev.app. You can also access the Help section within the app (Settings → Help).
Launching soon in the US and Canada (excluding Quebec)

See Where Your Body Is Heading.

Join the waitlist for launch news. Free to start, no extra hardware, and your data never leaves your iPhone.

Requires iPhone (iOS 16+) and Apple Watch Series 4+ · 18+

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