2019NEJM
Perez et al. (Apple Heart Study)
419,297 people
0.5% got an irregular-pulse alert. Among those who wore an ECG patch, 34% had atrial fibrillation, and 84% of alerts during the patch matched AF.
Find on PubMed ↗Research library · Tech & environment
Wearables and AI: what the data reveals
Watches, rings and AI models trained on millions of people are uncovering things doctors couldn't measure before: hidden heart rhythm problems, how many steps really matter, and the power of one-minute bursts of everyday activity.
Good evidence7ksteps a day linked to 47% lower death risk; benefits keep rising more slowly after thatDing 2025, Lancet Public Health, 57 studies
3 burstsof 1-minute huffing-and-puffing activity a day tied to about 40% lower death risk in non-exercisersStamatakis 2022, Nat Med, 25,241 adults
84%of watch irregular-pulse alerts matched atrial fibrillation on a simultaneous ECG patchPerez 2019, NEJM, 419,297 people
01
The basics
What your wearable actually measures
Most wearables shine green light into your skin and measure how much bounces back as blood pulses, a method called photoplethysmography (PPG). That gives heart rate, heart rhythm, HRV and estimates of blood oxygen. A motion sensor (accelerometer) counts steps and movement. Some add an ECG, skin temperature or breathing rate.
Accuracy varies. Steps and resting heart rate are good. Sleep duration is fair; sleep stages are rough estimates. Calories burned are often far off.
The big change is AI. Models trained on hundreds of thousands of ECGs, retinal photos or wrist-sensor recordings now spot patterns people can't see, such as signs of past atrial fibrillation in a normal-looking ECG. The best use for you: track your own trends over weeks, not single numbers.
How a smartwatch reads your pulse
Green light shines into your wrist. Blood absorbs more light with each heartbeat, so the reflected light pulses. AI models turn that wave into heart rate, HRV and rhythm alerts.
02
How it works
From wrist to insight
1
Sensor measures
Light and motion sensors record your pulse and movement all day.
2
Raw signal
A wave of blood flow and a stream of movement data.
3
AI finds patterns
Models trained on huge datasets turn signals into heart rate, sleep and rhythm alerts.
4
You see trends
Week-by-week changes are the most useful part.
03
Measure it
How much to trust each number
- Steps and resting heart rate: reliable for tracking trends.
- HRV: good if measured at the same time each night or morning. Compare only with yourself.
- Sleep: total time is fair; light, deep and REM splits are estimates.
- Calories: treat as a rough guide. Lab tests found errors of 27–93%.
- Irregular rhythm alerts: take them seriously and get an ECG from a doctor to confirm.
Try your number
LowGetting thereStrong benefitExtra, smaller gains
100015000
Ding 2025: risk falls steeply up to about 7,000 steps, then more gradually.
04
What the research says
The key studies
2019Lancet
Attia et al.
180,922 patients
An AI model spotted people who had atrial fibrillation from an ECG taken during a normal rhythm, with 79% accuracy.
Find on PubMed ↗2022Nature Medicine
Stamatakis et al.
25,241 non-exercisers, wrist trackers
Three short bursts a day of vigorous everyday activity (stairs, fast walking, carrying shopping) were linked to about 40% lower death risk.
Find on PubMed ↗2025Lancet Public Health
Ding et al.
57 studies
7,000 steps a day, compared with 2,000, was linked to 47% lower death risk and lower risk of heart disease, dementia, depression and falls.
Find on PubMed ↗2023JAMA
Khurshid et al.
89,573 adults, wrist trackers
'Weekend warriors' who packed their activity into 1–2 days got similar heart benefits to those who spread it out.
Find on PubMed ↗2017Journal of Personalized Medicine
Shcherbina et al.
60 adults, 7 devices
Most wrist devices measured heart rate within about 5%, but none estimated calories well; errors ranged from 27% to 93%.
Find on PubMed ↗05
Take action
Getting the most from your device
Use it as a coach for habits, not a judge of every day.
- 1
Watch weekly trends
Look at 7-day averages for resting heart rate, HRV, sleep and steps.
- 2
Act on rhythm alerts
An irregular-rhythm alert deserves a proper ECG with your doctor.
- 3
Add 1-minute bursts
Take stairs fast, walk briskly between meetings or carry the heavy bags.
- 4
Aim for 7,000+ steps
If you're at 3,000, adding 1,000 a day each month is a good pace.
- 5
Don't chase sleep scores
Worrying about the score can itself harm sleep. How you feel matters most.
- 6
Check privacy settings
Know who can see and sell your health data.
06
Myth or fact
Common myths, checked
✕ MythCalories burned on your watch are accurate.
✓ FactThey're often off by a quarter or more. Don't eat back 'exercise calories'.
✕ MythSleep stages are measured exactly.
✓ FactOnly a lab sleep study measures stages properly; wearables estimate them.
✕ MythMore data means better health.
✓ FactData helps only if it changes habits. Some people become anxious from tracking.
✕ Myth10,000 steps is a scientific target.
✓ FactIt began as a 1960s Japanese pedometer slogan. Benefits start well below it.
07
Questions
Frequently asked
Ring or watch?
Rings are good for sleep, HRV and temperature; watches add GPS, screens and ECG. Both measure trends well.
Should healthy people use a glucose monitor?
It can teach you how meals affect you, but isn't needed by most. See the blood sugar page.
Can a watch replace a doctor?
No, but it can tell you when to see one and give useful data to share.
08
Sources
References and related topics
- Perez et al. (Apple Heart Study) (2019). NEJM. 419,297 people. PubMed ↗
- Attia et al. (2019). Lancet. 180,922 patients. PubMed ↗
- Stamatakis et al. (2022). Nature Medicine. 25,241 non-exercisers, wrist trackers. PubMed ↗
- Ding et al. (2025). Lancet Public Health. 57 studies. PubMed ↗
- Khurshid et al. (2023). JAMA. 89,573 adults, wrist trackers. PubMed ↗
- Shcherbina et al. (2017). Journal of Personalized Medicine. 60 adults, 7 devices. PubMed ↗