Launching soon on the App Store

PubMed IDs, not vibes

The true line through the noise.

Spline reads what your iPhone and watch already record and turns it into one clear call each morning: push, ease off or rest. Every score shows what moved it and cites the study behind it. All of it stays on your iPhone.

See how it works

iPhone, iPad and Apple Watch · English and Deutsch · No account

  • 70+studies cited by PubMed ID
  • 555automated tests on the app’s logic
  • 0accounts to create
  • 0servers holding your data
Spline medium widget on the Home Screen: readiness 100, Prime, and today's call, Green light for Pull
Spline Today screen: the call Green light for Pull because HRV is 68 ms against a normal range of 57 to 67 ms, an alert about last night's two drinks, readiness 100 with its drivers, and day strain
Spline on Apple Watch: readiness 100, Prime, and today's call, Green light for Pull

Reads Apple Health, so it works with what you already wear.

  • Apple Watch
  • Oura
  • Garmin
  • Withings
  • Amazfit
  • Polardirect

Each device shares different data with Apple Health. Spline shows what it gets and says what’s missing, rather than filling the gap.

Today

One call each morning. With its reasons attached.

Spline compares last night with your own normal range, not a population average, and turns it into a decision you can act on before breakfast.

1

The call

Green light, ease off, swap to easy or rest, with the measurement that decided it right underneath. On a morning without enough data, it makes no call at all.

Today's hero card: Good afternoon, Alex. Green light for Pull. HRV 68 ms versus a normal range of 56 to 67 ms. Sleep regularity 95, over a 30-day recovery curve
2

What stands out

When one signal sits outside your usual range, like a short night, a raised resting heart rate or last night’s drinks, it’s named first. When nothing does, Spline stays quiet instead of cheering.

Needs attention: alcohol, 2 drinks, moderate dose, 0.36 grams per kilogram of pure alcohol
3

Why, in numbers

Every score opens into its drivers, each against your own baseline, and the formula that combined them. Tap the ⓘ on any metric for the method and its source.

Readiness 100, Prime. Drivers: HRV 68 ms, 11.5 percent above baseline; resting heart rate 48 bpm, 3 below baseline; sleep 7 hours 35 minutes

Screens from the current build, shown with demo data.

Features

Everything your data can tell you. Nothing it can’t.

Sleep, recovery, training, food and the long game. Each one built on a named method you can look up.

Sleep, and a plan for tonight

A sleep score built on regularity, duration and efficiency, the parts the research supports. Then tonight’s plan, worked back from your own nights: wind-down, last coffee, lights out.

Tonight: lights out 22:40 to wake at 6:55 with 7 hours 30 minutes asleep. Wind down 21:55, last caffeine 16:40 Sleep score 100, excellent: duration, efficiency and regularity each scored, with a note that stage percentages are deliberately left out

Your normal, not the population’s

HRV and resting heart rate measured against a 60-day range that’s yours alone. One low morning is a signal. A run of them is a pattern.

HRV versus your normal: 68 ms, above your 56 to 67 ms range, charted over 60 days against a shaded personal band

Every workout, analysed

Heart-rate zones, strain and recovery for each recorded session, with a share card that shows only what you choose.

A 55-minute easy aerobic run: 9.5 km at 5:47 per km, average heart rate 128, strain 6.8, with its heart-rate trace

Fitness, fatigue and form

A named training-load model, plus an HRV gate that can veto a hard day when your body disagrees with the plan.

Training status, productive: fitness 30, fatigue 35, form minus 5, with the form chart and a load ratio of 0.99, in range

Describe a meal. Check the numbers.

Type what you ate, scan a barcode or a label. Calories come from food databases or the label itself, never invented by the AI, and Spline learns from your corrections.

Check before logging, 408 kcal: eggs, sourdough, butter and a flat white, each matched to USDA or Open Food Facts with its portion source

Biological age, taken apart Pro

The age your vitals read as, with the years each factor adds or subtracts. Labelled as an estimate, because it is one.

Biological age estimate 18 against a calendar age of 28, with a chart of how many years VO2max, activity, sleep consistency, resting heart rate, HRV, sleep and body fat add or subtract

A monthly review from your own data Pro

A scorecard against the month before, what moved most, and a chart for every section. Ranked by how far each one moved.

Monthly review: biggest change, sleep up 17 minutes a night. Scorecard of sessions, hours, average sleep, HRV, resting heart rate and perfect days, then what moved most

Vitals, each against its normal

Blood oxygen, breathing rate, daytime stress and glucose. Home blood pressure is judged as a 7-day average against ESC 2024 thresholds, never on a single reading.

System today: blood oxygen 98 percent, respiratory rate 14.1 per minute, daytime stress 18 of 100, glucose 92 mg/dL

Breathwork, eyes shut

Cyclic sighing, the fastest evidence-backed way to downshift, with a haptic on every phase. Sessions log to Apple Health.

Cyclic sighing: double inhale, long exhale, shown as a rhythm strip of 3.5, 1 and 7 seconds, citing Balban 2023

Bloodwork, read on your iPhone Pro

Import a lab PDF or photo and it’s parsed on the device. Longevity-oriented ranges, and a test of whether a change is real or just noise.

  • PhenoAge
  • ApoB
  • Lp(a)
  • HbA1c
  • hs-CRP
  • Vitamin D
  • Omega-3 index
  • Testosterone
  • RCV change test

And the rest of your day

The quieter parts that make the numbers mean something.

  • Morning & evening routines
  • Non-negotiables
  • Humane streaks
  • Journal
  • Check-ins when a signal persists
  • Habit patterns, properly tested
  • Strength log & progression
  • Race countdown
  • Waist-to-height ratio
  • VO₂max & HR recovery
  • Peptide effect tracker
  • Printable health report
Ask Spline

Ask why. Get your numbers, with their sources.

Ask about your readiness, sleep or training in plain words. Spline answers from your own data and the research behind each metric, on your iPhone. If it hasn’t measured something, it says so.

  • The model picks the topic. It never writes the numbers. The on-device model only works out what you’re asking. The answer is built from the same values your cards show, so the two can’t disagree.
  • Every suggestion names its study. “Breathe slowly, daily” arrives with Laborde 2022, PMID 35623448. Change one thing at a time and let your own numbers judge it.
  • Works without the cloud. No chatbot server, no account. Without Apple Intelligence it still works, with a built-in keyword router.
Should I train hard today? What moves HRV? When was my HRV lowest last month?

Or ask Siri: What’s my call in Spline? It answers from your latest reading without opening the app.

Ask Spline is part of Spline Pro.

Ask Spline answering What moves HRV: three levers with published evidence. Build an aerobic base, Sandercock 2005. Breathe slowly, daily, Laborde 2022. Keep alcohol away from sleep, Pietilä 2018. Each with its PubMed ID
Beyond the app

Your call, wherever you look.

Widgets, your Apple Watch, Live Activities and Siri show the call your iPhone already made. It’s computed once, so they can never disagree.

Home Screen and Lock Screen

Readiness and today’s call at a glance. In the evening, the widgets switch to tonight’s lights-out time.

iPhone Home Screen with a medium Spline widget showing today's call, Green light for Pull, and a small readiness widget showing 100, Prime

On your wrist

The Apple Watch app and complications show the same call, clearly dated if your iPhone hasn’t updated it yet.

Spline on Apple Watch: readiness 100, Prime. Today's call: Green light for Pull

And the rest of iOS

  • Live ActivitiesSessions and breathwork count down on the Lock Screen and in the Dynamic Island.
  • Siri and ShortcutsAsk for today’s call, log food or start breathwork by voice.
  • Control CenterOne tap to log food, start breathwork or open your Brief.
  • iPadA multi-column layout that uses the whole screen.
  • Light and darkFollows your iPhone’s appearance, or pick one yourself.
  • English and DeutschFully translated, with an in-app language switch.
Evidence-first

Every number cites its science.

Other apps ship proprietary scores and ask for trust. Spline names the method behind each metric, links the study, and grades the evidence honestly, including where it’s thin. These are real citations from inside the app:

HRV-guided trainingVesterinen 2016PMID 26909534 Sleep regularity and mortalityWindred 2024PMID 37738616 Personal HRV baselinesPlews 2012PMID 22367011 PhenoAge blood clockLevine 2018PMID 29676998 Alcohol and overnight recoveryPietälä 2018PMID 29549064 Caffeine six hours before bedDrake 2013PMID 24235903 Adults need seven hours or moreWatson 2015PMID 26039963 Home blood pressure thresholdsESC guideline 2024PMID 39210715 Cyclic sighing trialBalban 2023PMID 36630953 Illness early warningCOVI-GAPP studyPMID 35728900 4×4 intervals and VO₂maxHelgerud 2007PMID 17414804 Waist-to-height ratioAshwell 2012PMID 22106927
Strong consistent trials or large cohorts Moderate good evidence, with caveats Emerging early or mixed, and labelled so Clinical a decision for your doctor

70+ studies cited across the app, each by PubMed ID, with the formula beside it.

Read every method
Built on rules

Six rules Spline won’t break.

Trust lives in the details. These aren’t slogans; they decide what the app is allowed to show you.

  1. No alert without its reason.

    Every warning carries the measurement that triggered it. The app has no way to raise one without it.

  2. Silence beats a false all-clear.

    Missing data is named, never filled in. With no HRV this morning, Spline makes no call rather than guessing one.

  3. Your normal, not an average.

    Baselines are your own 60-day ranges, so a “good” number means good for you, not for a stranger.

  4. “Associated”, never “caused”.

    Habit patterns are tested against the next morning, corrected for multiple comparisons, and reported as associations.

  5. The AI never owns a number.

    On-device models read your words and pick the topic. Every value comes from a measurement, a food database or the label you scanned.

  6. Your data is never paywalled.

    Export everything as JSON or CSV, free, forever. Pro pays for extra analysis, never for access to what you recorded.

Private by design

On your iPhone. Not on our servers. We don’t have any.

  • Computed on your iPhoneHealth data is read from Apple Health and analysed on the device.
  • No account, no sign-upOpen it and it works. There’s nothing to log into.
  • No analytics, no adsNo tracking SDK in the app, and this site sets no cookies either.
  • Take it with youFull JSON and CSV export, free and unconditional.
How Spline compares with a typical wearable app
 SplineTypical wearable app
Where the score comes fromNamed formula and PubMed IDProprietary, undisclosed
Where your data livesOn your iPhoneTheir servers
Hardware requiredWhat you already ownTheir band or ring
When the evidence is weakSays so, on the metricPresented as fact
Export your dataFree forever, JSON and CSVPartial, or none
Pricing

Free where it counts. Paid where it’s deep.

Your daily call, sleep, recovery, training and every export of your own data stay free, forever. Spline Pro adds the long-horizon analysis.

Spline

€0

Free, with no account and no trial timer.

  • Today’s call, readiness and its drivers
  • Sleep score, regularity and tonight’s plan
  • Recovery against your own 60-day range
  • Training load, session analysis and strength log
  • Food log: describe, scan or search
  • Body composition, waist, VO₂max and vitals
  • Routines, habits, journal and breathwork
  • Widgets, Apple Watch app and share cards
  • Every citation and formula
  • Full JSON and CSV export

Spline Pro

€3.99/ month

or €34.99 a year, about €2.92 a month (save 27%). All prices include VAT.

€0.99for your first month on the monthly plan, then €3.99. One introductory month per Apple ID.
  • Biological ageThe age your own vitals read as, the years each factor adds or subtracts, and where it’s heading
  • Bloodwork and lab importLab PDFs and photos parsed on-device; PhenoAge, ApoB and Lp(a); changes tested against the Reference Change Value; peptide effect tracker
  • Experiments and patternsN-of-1 experiments with effect sizes, bootstrap confidence intervals and false-discovery correction, plus habit patterns found for you
  • Adaptive planningRace-aware periodization and per-exercise strength progression targets
  • Monthly reviewA scorecard, what moved most, and a chart for every section
  • Ask Spline and shortcutsAnswers from your own data with cited levers, plus typed sentences turned into logged sets or a routine

Cancel any time in your Apple ID settings. Still no servers, no account, and no analytics on your health.

Payment is handled by Apple. Subscriptions renew automatically unless cancelled at least 24 hours before the end of the current period. See the Terms of Use and Privacy Policy.

What’s new

Recently added. Most of it this month.

  1. Sleep regularity, measured the published wayRebuilt minute by minute from your actual sleep, with night wakings and naps counted, as in the study it’s anchored to.
  2. A food log that learnsDescribe meals in English or German, with real portion sizes and better matching. Spline remembers your corrections, and My Foods holds the recipes the databases don’t have.
  3. Ask SplineAsk why a number moved or what to do about it. Answers come from your data, with the study behind each suggestion.
  4. Tonight’s plan, sessions and share cardsLights-out worked back from your own nights, now on widgets and your watch too. Every recorded workout gets a detail view, and days, sessions and months can be shared as images.
  5. Widgets, Live Activities and complicationsPlus a rebuilt Monthly Review, a lead signal on Today that names what stands out, and check-ins when a signal persists.
  6. Apple Watch, iPad and DeutschA watch app, a multi-column iPad layout, a full German translation, and Spline Pro.
Spline app icon

Stop trusting scores. Start reading them.

Spline isn’t on the App Store yet, but it’s close. Free to start, with no account, and your data stays on your iPhone.

Launching soon Read the methods first

For iOS 17 or later · iPhone, iPad and Apple Watch · English and Deutsch