The science behind the numbers

What each metric measures, how our sensors get it, and where it can go wrong. We think you should know the limits before you trust a score.

Heart rate
Heart rate variability
Blood oxygen
ECG
Irregular rhythm alerts
Sleep stages
Skin temperature
Respiratory rate
VO2 max estimate
Stress and recovery
Glucose
Blood pressure
Body composition
Cycle tracking
Fall detection

Heart rate

beats per minute

How fast your heart is beating, measured continuously or on demand.

What it is

The number of times your heart beats in a minute. Resting heart rate is the version worth watching: a lower resting rate usually tracks with better aerobic fitness, and a sudden rise of five or more beats for several days often means you are fighting something off, under-recovered, or dehydrated.

How it is measured

Almost every wearable shines green light into your skin and measures how much bounces back. Blood absorbs green light, so each pulse of blood shows up as a dip in the reflected signal. This is called photoplethysmography, or PPG. Chest straps measure the heart's electrical signal instead, which is more exact during hard exercise.

What to watch for

PPG struggles with tattoos, very dark or very cold skin, loose straps, and the arm motion of weightlifting or rowing. At rest it is close to clinical accuracy. During intervals, expect the wrist to lag a few seconds behind a chest strap.

Measured by

bits Ring, bits Band, bits Watch, bits Cuff, bits Scale, bits Pad

Heart rate variability

milliseconds

The small beat-to-beat changes in timing that reflect how rested your nervous system is.

What it is

Your heart does not beat like a metronome. The gap between beats shifts slightly with every breath and with the balance between your "rest and digest" and "fight or flight" systems. More variation at rest generally means more recovery capacity. Everyone's baseline is different, so the trend against your own average matters far more than the raw number.

How it is measured

The device times the gaps between pulses overnight, usually in the last few hours of sleep, and summarises them with a statistic called RMSSD. Rings and bands tend to get cleaner readings than watches because they sit still and press evenly.

What to watch for

A single night means little. Alcohol, a late meal, illness and a hard workout all drag HRV down for a day or two. Compare devices only against themselves; two brands can report numbers that differ by half and both be internally consistent.

Measured by

bits Ring, bits Band, bits Watch, bits Pad

Blood oxygen

percent saturation

How much of your blood's haemoglobin is carrying oxygen.

What it is

A healthy person at sea level sits between 95 and 100 percent. Overnight dips can hint at disrupted breathing during sleep, and the reading falls predictably at altitude. It is a screening signal, not a diagnosis.

How it is measured

The sensor adds red and infrared light to the green used for heart rate. Oxygen-rich and oxygen-poor blood absorb these colours differently, and the ratio gives the saturation.

What to watch for

Wrist readings are noisy compared with a fingertip clip. Motion, cold hands and a loose fit produce false lows. Treat a single low spot reading as a prompt to re-measure, not a result.

Measured by

bits Ring, bits Band, bits Watch

ECG

single-lead electrocardiogram

A 30-second recording of your heart's electrical rhythm, taken on demand.

What it is

An electrocardiogram traces the electrical wave that makes each heartbeat. The one-lead version on a watch or ring can tell a regular rhythm from atrial fibrillation, a common irregular rhythm that raises stroke risk. It cannot see a heart attack or most other conditions; that needs a 12-lead ECG in a clinic.

How it is measured

You touch the crown or bezel with a finger from the other hand, closing an electrical circuit across your chest. The device records for 30 seconds and classifies the result as sinus rhythm, atrial fibrillation, or inconclusive.

What to watch for

ECG features are regulated medical functions and are only switched on in countries where the maker has clearance. Inconclusive results are common if you move or your heart rate is high.

Measured by

bits Watch

Irregular rhythm alerts

background notification

Passive checks for signs of atrial fibrillation while you go about your day.

What it is

Unlike an on-demand ECG, these checks run in the background from the optical heart rate sensor. If the gaps between beats look irregular across several readings, the device prompts you to take an ECG or see a doctor.

How it is measured

The device samples pulse timing when you are still, several times a day and overnight, and looks for the chaotic spacing typical of atrial fibrillation.

What to watch for

Designed to catch people who do not know they have atrial fibrillation. It is not for people already diagnosed, and it will miss short episodes.

Measured by

bits Band, bits Watch, bits Cuff

Sleep stages

hours and stage breakdown

How long you slept and how the night split into light, deep and REM sleep.

What it is

Total sleep time and consistency are the parts you can act on. The stage breakdown is interesting but softer: a wearable cannot see brain waves, so it infers stages from heart rate, movement and breathing.

How it is measured

An accelerometer detects stillness, the optical sensor tracks heart rate and its variability, and the algorithm maps patterns onto sleep stages learned from lab studies. Bed sensors do the same from under the mattress using pressure and vibration.

What to watch for

Independent tests put stage accuracy in the 60 to 80 percent range against a sleep lab. Total sleep time is much more reliable than the deep or REM minutes. If a bad "sleep score" makes you anxious at bedtime, hide the score and keep the schedule.

Measured by

bits Ring, bits Band, bits Watch, bits Pad

Skin temperature

deviation from baseline

Overnight changes in skin temperature, used to spot illness and cycle phases.

What it is

Devices show the difference from your own baseline rather than an absolute number. A rise of half a degree for a couple of nights often precedes a cold. Across a menstrual cycle, the pattern of rises and falls tracks ovulation closely enough that some devices use it to predict periods.

How it is measured

A small thermistor sits against the skin and samples through the night, when room conditions and movement are stable enough to compare one night with the next.

What to watch for

A thick duvet, a hot room or a late workout can all move the number. Daytime readings are too noisy to be useful, which is why most devices only report overnight.

Measured by

bits Ring, bits Band, bits Watch

Respiratory rate

breaths per minute

How many breaths you take each minute while asleep.

What it is

Adults usually breathe 12 to 20 times a minute at rest. The number is remarkably stable from night to night, which is what makes a change worth noticing: a jump of a couple of breaths can be an early sign of fever or respiratory illness.

How it is measured

Each breath subtly modulates both heart rate and the optical pulse signal. The device extracts that slow rhythm from the overnight pulse record.

What to watch for

Only meaningful at rest. Snoring, sleep position and a partner moving the mattress can confuse bed sensors in particular.

Measured by

bits Ring, bits Band, bits Watch, bits Pad

VO2 max estimate

ml of oxygen per kg per minute

An estimate of your aerobic capacity, the best single predictor of long-term health that a wearable offers.

What it is

VO2 max is the most oxygen your body can use during hard exercise. Higher values track strongly with longevity. The lab test involves running to exhaustion while wearing a mask; wearables estimate it from the relationship between your heart rate and your pace.

How it is measured

Over several outdoor walks or runs with GPS, the device compares how fast you moved with how hard your heart worked, then adjusts for age and sex.

What to watch for

Estimates run within about 10 percent of a lab test for runners and are much rougher for cyclists, swimmers or anyone who rarely exercises outdoors. Watch the trend over months, not the number.

Measured by

bits Band, bits Watch

Stress and recovery

daily score

A composite score built from heart rate, variability, sleep and sometimes skin conductance.

What it is

Every brand packages this differently: Readiness, Recovery, Body Battery, Energy Score. All of them combine overnight heart rate, HRV, sleep and recent activity into a single number that suggests how hard to push today.

How it is measured

Mostly derived from the metrics above. A few devices add an electrodermal sensor that measures tiny changes in sweat gland activity, which rises with sympathetic nervous system arousal.

What to watch for

Scores reward lying still and punish late nights, so they can nudge you toward doing less. Useful as a gentle check, not a coach.

Measured by

bits Ring, bits Band, bits Watch

Glucose

mg/dL, every few minutes

Continuous readings of the sugar in the fluid between your cells, from a sensor worn on the arm.

What it is

Continuous glucose monitors were built for diabetes and are now sold over the counter to anyone. They show how specific meals, sleep loss and exercise move your glucose over the day. For people without diabetes the readings are interesting and occasionally useful for spotting large spikes, but the evidence that chasing a flat line improves health is still thin.

How it is measured

A hair-thin filament sits just under the skin for about two weeks and measures glucose in interstitial fluid. The reading lags blood glucose by 5 to 15 minutes.

What to watch for

No wrist device measures glucose, despite marketing that implies otherwise. Sensors cost money every two weeks, can fail early, and a reading above range after a meal is normal, not a problem.

Measured by

bits Patch

Blood pressure

mmHg

The pressure in your arteries, which only an inflating cuff measures reliably.

What it is

High blood pressure is the most common treatable risk factor for heart disease and stroke, and it has no symptoms. A home cuff that syncs to your phone is the single most useful piece of health tech most people over 40 do not own.

How it is measured

A cuff inflates on the upper arm and listens for the pulse returning as it deflates. Some watches estimate blood pressure from pulse wave shape after you calibrate against a cuff, and some show trends rather than numbers.

What to watch for

Watch-based estimates drift and must be recalibrated monthly. Hypertension notifications on recent watches flag a pattern over 30 days; they do not give a reading. Measure seated, rested, arm at heart level, and take the average of two readings.

Measured by

bits Cuff

Body composition

percent fat, muscle mass, water

How your weight breaks down into fat, muscle, water and bone.

What it is

Weight alone hides whether a change is fat or muscle. Composition shows the split, which matters more for health than the scale number.

How it is measured

A tiny electrical current passes through your body. Fat resists it more than muscle and water, so the resistance gives an estimate. Scales send the current foot to foot; a few add handles for a whole-body path.

What to watch for

Hydration swings the result by several percent, so measure at the same time of day, before eating. Trends across weeks are useful; day-to-day changes are mostly water.

Measured by

bits Scale

Cycle tracking

predictions and phase

Period and ovulation predictions improved by overnight temperature and heart rate.

What it is

Logging alone predicts periods about as well as a calendar. Adding overnight skin temperature lets a device confirm that ovulation happened, which tightens predictions and helps people with irregular cycles.

How it is measured

The temperature sensor detects the small, sustained rise that follows ovulation. Some devices also fold in resting heart rate and HRV, which shift across the cycle.

What to watch for

Retrospective confirmation of ovulation is not contraception. Illness and alcohol raise temperature in the same way.

Measured by

bits Ring, bits Watch

Fall detection

emergency alert

Detects a hard fall and calls for help if you do not respond.

What it is

The device notices the signature of a fall, waits for you to move or dismiss the alert, and otherwise contacts emergency services and your chosen contacts with your location.

How it is measured

Accelerometer and gyroscope patterns matched against a library of real falls, tuned to avoid triggering on a dropped arm or a hard tackle.

What to watch for

Needs a phone or cellular connection to place the call. Works best when enabled for people over 55; younger users can turn it on manually.

Measured by

bits Watch