Frequently Asked Questions

It depends on what you have. Most connections take under a minute, and you only do each one once.

iPhone and Apple Watch: Fuse reads Apple Health. Open Fuse, go to Connected Devices, tap Connect next to Apple Health, and allow the data types iOS asks about. Open Fuse once a day for the freshest data. Fuse also updates in the background when iOS allows it, and opening the app syncs right away. To change what Fuse can read later, open the Health app, tap your profile picture (top right), then Privacy → Apps → Fuse.

Android phone: Fuse reads Health Connect. Open Fuse, go to Connected Devices, tap Connect next to your Android health data, and allow the types Health Connect asks about. Whatever writes to Health Connect — your watch, your scale, Samsung Health — reaches Fuse the same way. Open Fuse once a day for the freshest data. Fuse also syncs in the background about once an hour.

Samsung Galaxy: see "How do I open Health Connect on a Galaxy?" above. You don't have to go through Samsung Health.

Oura, TrainingPeaks, Fitbit: open Fuse, go to Connected Devices, tap Connect next to the one you have, and sign in to that account when the window opens.

Whoop, Polar, Withings: you connect these the same way — tap Connect next to the one you have and sign in. We haven't yet confirmed data arriving in Fuse through these sign-in connections, so check Fuse a day after you connect. If nothing has shown up, tell us at hello@fuse.fit which one you connected. On Android, Google Fit workouts also reach Fuse through Health Connect.

Garmin: route it through your phone's health app for now. See "How do I get my Garmin data into Fuse?" below.

MyFitnessPal: this is not a password sign-in. Terra asks for your MyFitnessPal username (not your email) and a diary key — not your account password. Find your username under Settings → Profile in the MyFitnessPal app, or in the diary URL myfitnesspal.com/food/diary/USERNAME. Then set Diary Sharing to Locked with a Key under Settings → Diary Settings on the MyFitnessPal website (My Home → Settings → Diary Settings; the web page is the source of truth if the mobile toggle does not stick), copy that diary key, and enter username + diary key in the Fuse connect window. Terra's step-by-step: Why can't I connect MyFitnessPal?. MFP diary-sharing help: How do I make my diary visible to other users?.

Hevy or Intervals.icu: these use an API key you copy from their own app. Fuse shows you exactly where to find it — Connected Devices, then Direct Integrations. Hevy key: hevy.com/settings?developer. Intervals.icu athlete ID + key: intervals.icu/settings.

Strava: not supported, and that's a policy limit rather than a technical one. See "Does Fuse work with Strava?" below.

If your device isn't on this list, connect the phone instead — Apple Health and Health Connect between them carry data from most watches, rings and scales, including ones we don't connect to directly.

Every Fitbit, including Fitbit Air, connects the same way: through Google Health. In Fuse, open Connected Devices, tap Connect next to Fitbit, and sign in with the Google account your Fitbit uses. That one connection brings your Fitbit workouts, sleep and daily activity into Fuse.

If you connected Fitbit to Fuse the old way, reconnect once the same way. The old connection stops delivering new data. Everything already in Fuse stays.

Android Health Connect is a separate phone path. It is not the same as this connection.

Some wearable makers have set limits on how their data can be used with AI features. We are reviewing those terms, and we will let you know if anything changes for you.

If something you'd expect from your Fitbit is missing after you reconnect, let us know at hello@fuse.fit.

You can connect your Garmin to Fuse today through your phone's health app: Apple Health on iPhone, Health Connect on Android. A direct Garmin connection is coming later. We don't have a date for it yet. The phone path carries fewer metrics than a direct Garmin connection would.

What comes through (what we've seen so far):

• iPhone (Apple Health): heart rate, resting heart rate, sleep with stages and calories — we're still confirming this list on iPhone. Steps from a Garmin may not reach Fuse on iPhone. Workouts arrive with calories but without heart-rate detail.
• Android (Health Connect): resting heart rate, steps, sleep and workouts. We haven't confirmed sleep stages on this path yet.
• Not confirmed on either path yet: HRV, Garmin's sleep and recovery scores, Body Battery, blood oxygen, and workout GPS and heart-rate detail.

Anything that doesn't arrive won't appear in Fuse. If a Fuse score looks thinner than usual, this is the likely reason.

Some wearable makers have set limits on how their data can be used with AI features. We are reviewing those terms, and we will let you know if anything changes for you.

Keep your Garmin. Route it through your phone's health app, then connect that app to Fuse.

1. In the Garmin Connect app, open settings and turn on sharing to Apple Health (iPhone) or Health Connect (Android). Allow all the data types it offers.
2. In Fuse, go to Settings → Connected Devices, find the Apple Health (iPhone) or Health Connect (Android) card and tap Connect.
3. Open Fuse once a day for the freshest data. Fuse also syncs in the background (on iPhone, when iOS allows it), and opening the app syncs right away. Give it a sync or two before judging what's missing.

If something the list above says should arrive is missing, tell us at hello@fuse.fit and say which phone you use.

No. Strava's API policy doesn't allow its data to be used with AI applications, so we can't support a Strava connection, either directly or through a third-party sync service.

No new Strava data will sync.

The good news: most of what Strava records reaches Fuse anyway if your watch is connected. Apple Health and Samsung Health both send workouts and heart rate to Fuse, so if your watch writes its workouts and heart rate into one of them, that data reaches Fuse too — Strava is usually a mirror of that same data, not the source of it. Not every watch writes both to Apple Health or Samsung Health; check your watch app's health-sharing settings. Garmin is a partial path right now; see "What works with Garmin right now, and what doesn't?" below.

If you record workouts only on Strava and don't have a watch connected to Fuse, you can still keep your training score current by logging activities manually. Go to Enter Data Manually to log a run, ride, or session by type and duration — no device connection required.

Strava's API policy: strava.com/legal/api_policy

Yes. The Hume Body Pod writes weight and body fat to Health Connect. If you have Health Connect set up in Fuse, your weight flows in automatically — no separate Hume account or login needed.

To enable it: open Health Connect on your Android phone, go to App access → Hume, and turn on Allowed to write for Weight. Fuse uses weight only today; body fat and body measurements from Hume do not reach Fuse yet. On Samsung phones: Samsung Health → Settings → Health Connect, then grant Hume write access.

Samsung Health reads Hume body-composition data only if Health Connect has the right write permission set for Hume. Open Samsung Health → Settings → Health Connect, find Hume in the app list, and confirm Allowed to write is on for Body fat and Body measurements. Once that permission is active, your next Hume weigh-in will write to Health Connect, and Fuse picks up the weight on the next sync. Fuse does not yet use body fat or body measurements from Health Connect, so those stay in Samsung Health for now.

You don't have to go through Samsung Health. Open Phone Settings and search Health Connect. On many Galaxy phones it sits under Other. That's the same Health Connect as Samsung Health → Settings → Health Connect. Confirm two things: Samsung Health is writing sleep, heart rate, and steps, and Fuse is allowed to read those same types. Opening Fuse is what sends the data — the Health Connect screen itself does not.

Think of your Fuse Score as a daily battery. It charges from inputs (sleep, recovery, nutrition, mental health) and drains from outputs (training, stress, activity). Fuse Coach reads the fused picture and tells you what is driving the number today.

You do not have to pick a single device. Connect what you already use — wearables, strength apps, food logs — and Fuse merges them into one read. Subjective check-ins are part of that same picture, not a side note. See "Why do subjective check-ins matter?" below.

The Dashboard is your home screen: Fuse Score, a check-in prompt, Coach's Take, and the tiles you care about most. Tap a tile for a deeper breakdown.

Tap Tap to arrange to choose which tiles show, and use the metric picker on a tile to swap what it tracks. The full breakdown of every input, output, and health chart lives on the Input, Output, and Health pages — the Dashboard stays the daily front door.

Input is everything that charges the battery: sleep, recovery, nutrition, and related recovery signals. From Input you can open Nutrition and Recovery for charts and logs. Food you sync (for example from MyFitnessPal) and recovery metrics from your devices show up here.

Output is training and activity — how much work you did, and how that work lines up with the goals you set at onboarding. Cardio and strength stay in separate lanes: a Hevy lifting session feeds Strength Output; runs and rides feed cardio. From Output you can open the Activity Journal for the session list.

Health compares you to science-backed norms for people like you (age × sex demographic baselines), not only to yesterday's workout. From Health you can open Flights (log travel and related sleep) and Body composition.

Several logs feed the same engine from different angles:

• Mental health / how you feel — meditation, breathing, rest, stress, mood, energy, motivation, and related check-in signals.
• Food — meals and nutrition (synced or logged).
• Flight log — trips and time-zone changes so recovery is not judged as if you never traveled.
• Workout / activity journal — sessions from devices and apps.
• Body composition — weight and related measures from a scale or Health Connect.

You do not need every log every day. The ones you use regularly are what make Coach's read personal.

Fuse keeps cardio and strength in separate lanes so a lifting day does not get scored as if it were a run (and the reverse).

• Cardio / training output — runs, rides, and other aerobic sessions feed the Output score against the goals you set at onboarding, not only "how much work you piled on."
• Strength load — volume from a strength tracker (for example Hevy) stays in its own strength lane and is not mixed into cardio output.

Why separate them: heart rate and duration are a poor measure of how hard lifting is, so mixing strength into cardio zones or cardio output would misread the day. Use your connected strength app for lifts; keep watch/GPS sessions for cardio. (See also "How do Input, Output, and Health scores differ?" and heart-rate zone notes on strength sessions.)

Yes.

• Dashboard, Input, Output, and Health: use Tap to arrange on each page's tile grid to reorder tiles, then tap Done. You can also change which metrics fill a tile.
• Help / FAQ: open Settings → FAQ (top tile) or Settings → Feedback → Help Center. Help Center groups answers by topic (Getting Started, How Data Syncs, Devices & Integrations, Device Guide, Your Account) — expand any topic to read it, and jump between topic groups the same way you pick which tile pages to customize above. (Dashboard, Input, Output, and Health use Tap to arrange on their tile grids; Help Center is topic navigation over curated Q&A.)

Tell Fuse Coach in chat that you traveled, and log the flight on the Health → Flights page when you can. Fuse adjusts how hard it judges recovery during the catch-up window instead of treating jet lag like a normal bad night. (Same guidance as Help Center → How Data Syncs.)

A workout without heart rate is almost never something you did wrong — it usually means the data never reached us. There are three reasons it happens, and they have different fixes.

Your source isn't connected or isn't permitted. Heart rate reaches Fuse through whichever device recorded the session. If that device isn't connected, or the health permissions for heart rate aren't granted, the workout arrives as duration only. Check the integration in Settings, and on Android confirm Health Connect allows Fuse to read heart rate.

You recorded the session on something that wasn't measuring heart rate. A phone-tracked walk or a manually logged session has no heart-rate source behind it. Logging the session still counts toward your training — it just won't carry the heart-rate detail.

The device doesn't send heart rate during workouts. Not every device or connection reports continuous heart rate for a session, even when it reports heart rate elsewhere during the day. Where that's true for a specific device, we say so — we're building that per-device list from our own measured ingest data rather than from vendor marketing claims, and it will appear here as we confirm each one.

With a free TrainingPeaks Basic account, TrainingPeaks shares a summary only. That is the workout type, time, duration and distance. Heart rate, power, calories, TSS and workout detail need TrainingPeaks Premium.

You don't need TrainingPeaks to use Fuse. If you use Android and a Garmin watch, connect Garmin through Health Connect. That path gives Fuse the most detailed heart rate from your watch. TrainingPeaks is optional, and on a Basic account it adds no detail Fuse doesn't already get from your watch.

If a watch connected directly to Fuse also recorded the workout, Fuse can use the watch's record. That record includes the heart rate.

If you connect TrainingPeaks as well, Fuse attaches your watch's heart rate to the TrainingPeaks workout by time. That works whether the watch reaches Fuse directly or through Apple Health or Health Connect. It needs workout heart rate from the watch for that time. Background readings taken several minutes apart are not enough.

If a workout is missing heart rate you'd expect to see, tell us at hello@fuse.fit.

Chest straps that measure the electrical signal of your heart (ECG) are more accurate during exercise than optical sensors on the wrist or finger, which are more affected by movement. If you train with an ECG chest strap, setting it as your workout source in Settings tells Fuse to prefer it for your sessions instead of averaging it with a less accurate reading.

This matters most for intervals and anything with rapid changes in effort, where optical sensors tend to lag. It applies to any session you wear the strap for — including mobility and stretching work, not just hard cardio.

If you don't use a chest strap, leave the setting alone. Fuse picks the most accurate source it has for each session on its own.

Fuse works out your five zones from your own numbers, not from a chart. Two inputs matter: your maximum heart rate and your resting heart rate. Your resting heart rate comes from your device each day. Your max starts as an age-based estimate, and once Fuse has seen a few genuinely hard workouts from you, it switches to the max it has actually observed in your own data — which is more accurate than any formula. From those two numbers Fuse sets your zone boundaries using the heart-rate-reserve method: it looks at the range between your resting and max heart rate, and places each zone as a slice of that range. Two people with the same max but different resting heart rates get different — correctly different — zones.

While Fuse is still estimating from age, your zones are labeled as estimates, and they can be off by around ±10 beats. One hard cardio session is the fastest way to sharpen them.

You won't see zones on strength, rehab, or mobility sessions. That's deliberate: heart rate is a poor measure of how hard lifting is (it climbs for reasons that have nothing to do with aerobic effort), so showing zone credit for a squat session would be telling you something false. You still see your average and peak heart rate for those sessions.

That automatic setup is the default, not the ceiling. In Settings → Training you can choose your zone method — heart-rate reserve or lactate threshold — and enter your own measured numbers, and every zone in Fuse will follow your choice. The questions below explain what the methods are, how to pick one, and how to get the numbers each one needs.

By the science, there's a clear ranking:

1. A real threshold test. Zones anchored to your lactate threshold — measured in a lab, by a watch that runs a guided threshold test, or with the 30-minute field test described below — are the gold standard, because the threshold is a real, measured feature of your body, not a percentage of anything. In Fuse, pick the Lactate Threshold method in Settings → Training and enter that number.
2. Zones computed from your measured max and resting heart rate. This is what Fuse does automatically. It's the best method that doesn't require a test.
3. Age formulas. "220 minus age" and its cousins miss the mark by ±10 beats or more for a typical person. This is what most devices use out of the box.

The uncomfortable truth about defaults: the most common setup on people's wrists — zones as percentages of an age-estimated max — is the least accurate of the three. Fuse never leaves you there once it has your data.

It depends on where your watch's zones came from.

If you've done a threshold test — for example, some Garmin watches can measure your lactate threshold during a guided run and set your zones from it — those zones are anchored to your actual physiology, and they're better than anything computed from max and resting heart rate alone. Use them: pick the Lactate Threshold method in Settings → Training and enter the threshold heart rate your watch measured, and Fuse will build every zone from your number instead of its own.

If you've never done a threshold test, your watch is almost certainly showing its factory default — usually percentages of "220 minus age" — and Fuse's automatically computed zones will be more accurate. Let Fuse do it.

Fitbit and Fitbit Air are a version of that same factory default, not a threshold test. Per Google's current Fitbit help documentation, Fitbit's zones are set from a fixed 220-minus-age max and heart-rate-reserve percentages, and the only zone-related setting you can adjust yourself is an inactivity alert threshold — not the zone boundaries. Fuse computes your zones from your own raw heart-rate data — your observed max and your measured resting heart rate — across whatever devices you connect, so on Fitbit or Fitbit Air, Fuse's zones are the more personalized numbers.

One honest limitation: watches don't send their zone settings to Fuse through our current data connections — we've verified this for Garmin, Oura, and Whoop, and the list of devices we've checked is growing. Some send how long you spent in their zones, but not the zone boundaries themselves. So if you want Fuse to use your watch's threshold-based zones, entering them in settings is the way to do it today.

Also expect Fuse's zones to differ from your watch's — each brand calculates zones its own way (different formulas, different bands), which is exactly why Fuse computes one consistent set across all your devices.

A heart rate zone is a band of effort, marked out in beats per minute. Zones are not a score, and a higher zone is not a better zone — each band is a different training tool. The low zones build your aerobic base and let you accumulate a lot of training without wearing yourself down; the high zones sharpen speed and top-end fitness and are only sustainable in small doses. Exercise science writes its training guidance in these bands: the American College of Sports Medicine, for example, defines "moderate" and "vigorous" exercise as specific slices of your heart-rate reserve (Garber et al., 2011, ACSM Position Stand).

Zones are personal. Two people running side by side at the same pace can be in different zones, because a zone is defined by your physiology — your maximum, resting, or threshold heart rate — not by pace or by how hard the effort looks from outside. That's why Fuse asks for your numbers instead of reading them off a chart.

Because the estimate everyone knows — "220 minus your age" — is wrong by a lot, for a lot of people. Researchers who traced the formula's history found it was never derived from a rigorous study of maximum heart rate, and that formulas of this type carry a standard error on the order of ±10 beats per minute — which means roughly one person in three is off by more than 10 beats, and some by 20 or more (Robergs & Landwehr, 2002, Journal of Exercise Physiology online 5(2); Tanaka, Monahan & Seals, 2001).

A wrong ceiling doesn't produce one wrong number — it shifts every zone, because every boundary is computed from it. We've measured this with real data: one of our own testers had 185 stored as his max while his true max is 200. Every zone boundary sat about 7% low, and his easy sessions displayed as tempo work — the app was effectively telling him to back off on days he was training exactly right.

So Fuse treats formulas as a last resort, asks you for measured numbers where you have them, and gives you honest ways to get them where you don't — see "How do I find my resting heart rate, max heart rate, and LTHR?" below.

Fuse's zone calculator offers two computed methods (plus fully custom bounds if you already have zones you trust).

Heart-rate reserve (also called the Karvonen method) places five zones across the range between your resting and your maximum heart rate — your "reserve." It needs two numbers: max HR and resting HR. Because it accounts for your resting heart rate, it tracks your actual relative effort more faithfully than percentages of max alone (Karvonen, Kentala & Mustala, 1957; Swain & Leutholtz, 1997), and both numbers are easy to get — your wearable measures resting HR every night, and max can be observed from your own hard sessions.

Lactate threshold builds seven bands as percentages of your lactate threshold heart rate (LTHR) — roughly, the highest effort you can hold steadily for about an hour (Friel's zone guide on TrainingPeaks). Its strength is that the anchor is a real physiological event you measured, not an estimated ceiling, which is why it's generally the preferred method for trained endurance athletes. Its cost is that you have to measure it: a hard 30-minute field test, or a lab or watch-guided threshold test.

How to choose, honestly: a measured reserve setup beats a guessed threshold setup. If you haven't done a threshold test and don't want to, choose Reserve with real numbers — it is a good method, not a consolation prize. Choose Lactate Threshold when you have an actual measured LTHR, or you're willing to do the field test and re-test as your fitness changes.

One thing worth knowing: zone models genuinely disagree with each other. Researchers often work in three zones (Seiler & Kjerland, 2006), most consumer devices show five, and Friel's threshold system uses seven bands — and the exact boundary percentages differ between authors and brands. There is no single canonical banding. What matters most is that your zones are anchored to your own measured numbers and used consistently, which is what the calculator does: Fuse displays the band structure of whichever method you choose, the same way everywhere.

Resting heart rate — the easy one. If you wear a device that measures overnight or morning resting heart rate, that's your source; the calculator's "Suggest my values" button reads it from your recent days and tells you where the number came from. Measuring by hand also works: first thing in the morning, before you get out of bed, count your pulse for a full 60 seconds; do it a few mornings and use the typical value (Harvard Health on resting heart rate).

Max heart rate — formula last, data first. Age formulas are the last resort (see above — typical error around ±10 beats). Your real max is the highest heart rate you actually reach in a genuinely hard effort, so if Fuse already has hard sessions from you, "Suggest my values" will offer the max observed in your own data, labeled with its source — no special test needed. A deliberate field test, if you want one, looks like: a long warm-up, then several minutes of progressively harder effort (a steady hill or repeated hard intervals) taken close to your limit — the highest reading is a good approximation of your max. A word on safety, because a max test is by definition an all-out effort: if you're new to vigorous exercise, returning from a long break or illness, or have a heart condition or symptoms like chest pain or dizziness during exercise, talk to a clinician before doing a maximal test — that's the standard pre-participation screening guidance (Riebe et al., 2015, ACSM). The observed-from-your-data suggestion requires no test at all.

LTHR — the 30-minute field test. The standard protocol comes from Joe Friel: ride or run a 30-minute time trial alone (not in a race, not with training partners), at a steady effort as if racing the full 30 minutes; press the lap button 10 minutes in; your LTHR is your average heart rate for the final 20 minutes (Friel's zone guide on TrainingPeaks). The most common mistake is starting too hard and fading — that skews the number, so pace it steady. Test per sport if you train both running and cycling; the two thresholds differ. Honest footnote: Friel's own site also describes a shorter variant — a 20-minute time trial, taking the whole-test average minus 5% (joefrieltraining.com). Both are approximations of the same threshold; the 30-minute version is the one our instructions follow. If you'd rather not test yet, the calculator's "Estimate from my data" button scans your stored workout heart-rate history for your hardest sustained 20-minute effort — the same statistic the field test measures — and tells you how confident it is in the result.

Because each brand defines "resting heart rate" a little differently. The devices aren't disagreeing about your heart. They're measuring different moments.

• Oura reports your lowest and average readings from your time in bed (Oura's explainer).
• Garmin typically reports the lowest readings of the day, which usually fall during sleep.
• Fitbit describes resting heart rate as your heart rate when you're still and well-rested, estimated each day. It treats sleeping heart rate as a separate measure (Fitbit's Help Center).

So two devices can differ by several beats per minute, and each is doing what its brand intends.

The trend on one device is more useful than the gap between devices. If your resting heart rate on one device shifts over a few weeks, compare it with that same device's earlier readings. It doesn't need to match another device's number. Fuse is for general wellness information and isn't a medical device. Talk to a healthcare professional about any health concerns.

Keep your device. Fuse fuses it.

Everything that touches zones follows your configuration — and it all agrees with itself:

• The zone ranges you see and the minutes we count come from the same numbers. The beats-per-minute range printed next to each zone is exactly the range used to classify your time-in-zone, on the Output card, the cardio charts, and everywhere else.
• You see your bands, in beats per minute. Heart rate is measured in bpm in both unit systems, so nothing changes if you switch between Imperial and Metric. If you chose the Lactate Threshold method you'll see its seven bands where there's room in the display; compact charts group the three top sub-bands into one Zone 5.
• Your history follows your current method. Zones are a lens over your recorded heart-rate data, not part of the recording — so when you change method or update a number, past days are re-read under the new zones too. Your charts reflect your current method, which is why a session can move zones after you update your numbers: the data didn't change, your zones got more accurate.
• Suggested values are labeled. Anywhere Fuse suggests a number for you, it tells you whether it was observed in your data, read from your device, or estimated from a formula — so you always know how much to trust it.

The usual signs: sessions that felt easy and conversational register as tempo (Zone 3), or you can't reach the top zone even in an all-out effort. Both usually mean one input is off — most often a max or threshold that no longer matches your body.

Three steps:

1. Re-check the inputs, not the zones. Open Settings → Training and look at the numbers behind your method. Resting heart rate drifts with fitness, stress, and sleep, and training itself lowers resting heart rate over time — that was the finding of the original Karvonen study (Karvonen, Kentala & Mustala, 1957). A resting value from months ago may simply be stale.
2. Re-measure or re-estimate. Use "Suggest my values" or "Estimate from my data" to pull fresh numbers from your recent history, or re-run the field test for the number you suspect (see the question above). If your training has changed markedly — much fitter, or coming back from time off — a re-test is the honest fix; thresholds and resting heart rate move with fitness in both directions.
3. Save, and let everything recompute. Updating a number updates every zone surface, including your history, so the fix is visible immediately — you don't need to re-log or edit anything.

If you've re-tested and something still looks off, tell the coach what you saw — a session name and what felt wrong is enough for us to check the data behind it.

Mood, energy, stress, sleep quality, motivation, and how you feel are treated as biometrics alongside device numbers. Fuse compares three signals: what your devices report, what you say in check-ins, and how your actual workouts went. When devices say you are fresh but you feel drained — and your training output backs that up — Fuse smooths toward the subjective signal, because the device is missing context your body is reporting.

A short check-in with Fuse Coach (or the guided prompts on the Dashboard) is enough; you do not need a separate form for every day.

• Input — how well you fueled and recovered (sleep, nutrition, recovery, mental health).
• Output — whether your training matches the goals you set at onboarding, not only how much work you piled on.
• Health — how you trend against demographic norms for people like you, not only against yesterday's session.
• Fuse Score — the live battery across the day: charges from inputs, drains from outputs.

Strength volume stays in its own lane and is not mixed into cardio output. Use a strength tracker such as Hevy if you lift; runs and rides keep feeding the cardio side.

They measure different things. Apple Watch reports SDNN, which looks at how much your heartbeat timing varies overall during short readings taken while you're still. Oura, Whoop, Garmin and Fitbit report RMSSD, which looks at beat-to-beat changes and is measured while you sleep. Both are real measurements of heart rate variability, but they are on different scales, so the numbers are not expected to match. Fuse tracks each one against its own history and never converts one into the other.

Each brand measures over a different part of the night and uses its own sensor and processing. Fitbit measures during your main sleep, and one of its values covers deep sleep only. Oura averages five-minute readings across your whole sleep. Garmin averages overnight. Polar uses roughly the first four hours of sleep. Different windows and sensors give different numbers, and a steady gap between two devices — in either direction — is normal. What matters is whether each device's number is higher or lower than its own usual range.

Fuse keeps Apple's SDNN separate from RMSSD devices (Oura, Whoop, Garmin, Fitbit, Polar) and labels which one you're seeing. If you wear more than one RMSSD device, Fuse is working on showing each device's own number rather than one blended figure — that update is coming. Today, compare the number Fuse shows against your own recent trend rather than against any other app's number for the same night.

Averaging numbers that are on different scales produces a number that no device measured and that doesn't match any app you use. Fuse is built to compare each device to its own history instead — the approach used in sports-science research on HRV monitoring — and is rolling that out across every device combination.

HRV naturally moves from night to night with sleep, alcohol, illness, travel, stress and training. A single night means little. Fuse looks at your recent trend rather than any one reading, because that's more reliable. HRV is a wellness indicator, not a diagnosis. If you have health concerns, talk to a doctor.

Not meaningfully. HRV varies widely between people, and between devices for the same person. Fuse compares you only to yourself.

Two different clocks are involved.

1. Getting the data to Fuse. Your watch or app syncs to its own cloud first. Fuse's sync partner (Terra) then delivers it to us. That vendor-side wait is what users usually notice — often seconds, sometimes several minutes, occasionally longer if the device has not synced yet. Opening the device's own app (or unlocking your phone and opening Fuse for Apple Health / Health Connect) is often what finally pushes the day across.

2. Computing the number you see. Once a payload has arrived at Fuse, measured receipt-to-processed time across six providers is 0.58–0.84 seconds (p50, measured 2026-09-21). That is not the multi-minute wait. The wait you feel is almost always the device → cloud → Terra → Fuse leg, not Fuse "thinking."

On the Fuse Score and related tiles you will see a small Computed … ago (or Computed just now) line. That is when this number was last refreshed from the data Fuse already has — so you can tell a stale score from a live one without guessing.

Open Fuse once a day for phone-based sources. Opening the app is what pulls your latest Apple Health or Health Connect data across; sitting on the Health Connect screen alone does not send anything.

If a whole day is still missing after you have opened Fuse and confirmed the source app recorded it, write to hello@fuse.fit with the date — we can see what arrived and what did not.

Open the Fuse app. That is genuinely the first step: opening the app is what pulls your latest data across, so a phone that hasn't had Fuse opened in a few days looks exactly like a broken connection when it isn't.

If your data is still behind after opening Fuse, check that the app you sync from has actually recorded the day — Apple Health, Health Connect, or the device's own app. If it has and Fuse hasn't caught up, write to us at hello@fuse.fit and tell us the date you're missing. We can see what arrived and what didn't.

Your devices don't all record time the same way. Some send us the time in your local clock along with which time zone they think you were in; others send the time in a single world-standard clock and leave the time zone to us. Most of the time both arrive at the same answer.

Occasionally a device gets the time zone part wrong — it records the right clock time but labels it with the wrong zone. When that happens the workout can land a few hours off, and if it was late in the evening or early in the morning it can even land on the neighbouring day.

Nothing is lost when this happens, and it doesn't change how hard the workout counted. If you spot a session sitting at a time you know you weren't training, tell the coach — mentioning the day and roughly when you actually trained is enough for us to line it up.

If two of your devices both recorded the same session, and they disagree about what time zone you were in, we can end up treating one workout as two. It's the same cause as the question above: the two records look hours apart even though they describe the same hour of your day.

You don't need to delete anything. Tell the coach which session was doubled and we'll merge it.

If no authorization screen appeared, the reconnect did not take. Open the Whoop app → Profile → Connected Apps → revoke Terra → return to Fuse → tap Reconnect again. This is a known Whoop-side quirk, not a Fuse login failure. (Same steps as Help Center → How Data Syncs.)

A wearable connection is an authorization between Fuse's sync partner and your device account. It can quietly expire or be revoked on the device's side — for example after you changed that account's password, or you revoked access inside the device's own app. Fuse then shows reconnect needed. Re-linking under Connected Devices restores the flow; your Fuse history is not deleted. (Same framing as Help Center → How Data Syncs.)

Oura connections can show Stale and need a one-tap reconnect from Connected Devices. Other users' Oura can keep flowing at the same time, so a single stale Oura is usually account-specific, not a global outage. If the authorize screen stalls, try again later or write to hello@fuse.fit. (Same guidance as Help Center → How Data Syncs.)