Hello there! Ever wondered about that trusty device on your wrist? Smartwatches, fitness trackers, and other wearables have become incredibly popular, claiming to do everything from counting your steps to monitoring your heart and sleep—thanks largely to built-in tools like the heart rate sensor. These gadgets aim to help you quantify your overall health and wellness. But with so many features and promises, a big question often pops up: Can you really trust the numbers these devices give you?
You’re not alone if you’ve pondered this. This blog post is for anyone who uses these devices, is thinking about getting one, or simply wants to understand how these little gadgets work—especially the heart rate sensor, which is central to many fitness and health metrics—and how accurate they truly are. We’re going to dive into the science, look at some real-world tests, and help you figure out if your wrist-mounted health tracker is as useful as it claims to be. Let’s find out together!
Unpacking the “Magic”: How Smartwatches Track Your Health
At their core, smartwatches are pretty sophisticated mini-computers packed with various sensors that collect data about your body and your surroundings. Most of their functions, especially health monitoring and exercise tracking, rely heavily on these integrated sensors and clever algorithms.
Here’s a breakdown of how they do what they do:
Counting Your Steps: Accelerometers and Gyroscopes
Probably the most checked number on any smartwatch is your step count. How do they do it?
- Accelerometers are tiny electronic gadgets that measure acceleration. Essentially, they detect if you’re moving and how. By identifying patterns in this raw motion data, smartwatches can count your steps and even estimate distance and calories burned. They can also tell the device’s orientation, which helps the display wake up when you raise your wrist.
- Gyroscopes work alongside accelerometers to monitor your body’s orientation and angular movements. Combining both sensors helps smartwatches get a more accurate picture of your movement and can even help save battery life by turning off the display when you’re still.
However, tracking steps from your wrist can be a bit tricky! A step counter on your waist gets a clearer signal, but your arm might move for all sorts of reasons not related to walking, making the signal harder to interpret.
Monitoring Your Heart: PPG and ECG
Your smartwatch tracks your heart using two main technologies, each powered by a heart rate sensor designed to decode what’s happening beneath your skin.
Photoplethysmography (PPG)
This is the most common type of heart rate sensor used in smartwatches. While the term might sound complicated, the science behind it is surprisingly straightforward:
- Light at Work: The watch shines green LED lights onto your skin. Why green? Because blood absorbs green light very efficiently—thanks to the color contrast between red (blood) and green.
- Detecting Blood Flow: As your heart beats, blood volume in your capillaries fluctuates. These changes affect how much green light is absorbed versus reflected back.
- Reflected Light and Algorithms: The optical heart rate sensor on the back of your watch captures the reflected light. Built-in algorithms then analyze these subtle changes to calculate your heart rate accurately.
- The “Flashing” Secret: Those green flashes you see? That’s your watch taking a reading. Between flashes, it checks for ambient light interference—like room lighting or sun glare—and subtracts it to improve accuracy. Green light also strikes a balance: it penetrates deeply enough to track blood flow, without picking up noise from nearby muscles.
Electrocardiogram (ECG)
Some advanced smartwatches include an ECG feature, which offers another method for heart monitoring.
- Electrical Activity: Unlike PPG, ECG doesn’t rely on light. Instead, it records the electrical impulses your heart generates with each beat.
- How It Works: Clinical ECGs use multiple electrodes and gels for high-precision readings. In smartwatches, ECG functionality typically uses two touchpoints—one on the watch and another where you place your finger. Though not as accurate as a hospital-grade ECG, it’s capable of detecting heart irregularities like atrial fibrillation (AFib).
Measuring Blood Oxygen: SpO2 Sensors
Your smartwatch can also measure your blood oxygen saturation (SpO2) levels, which indicates how well your heart circulates blood to your lungs and the rest of your body.
- Red and Infrared Light: This works similarly to PPG but uses red and infrared (invisible) lights.
- Oxygenated vs. Deoxygenated Blood: Oxygenated blood cells absorb more infrared light and allow red light to pass through, while deoxygenated blood cells absorb more red light and allow infrared to pass through.
- Ratio Calculation: By measuring the ratio of these two types of light reflected back to the watch, the device calculates your SpO2 level, typically displayed as a percentage.
How Smartwatches Track Your Health and Fitness
Smartwatches, fitness trackers, and wearables are devices that aim to quantify your health and wellness by tracking various metrics such as steps, heart rate, and sleep. Many people globally are using fitness trackers nowadays. These devices use different sensors and algorithms to collect and interpret data.
Heart Rate Monitoring (HRM)
How It Works
Most smartwatches rely on a built-in heart rate sensor using photoplethysmography (PPG) technology. This involves:
- Shining green LED light onto your skin.
- Measuring the reflected light to detect changes in blood volume.
- Interpreting these fluctuations as heartbeats using software algorithms.
The heart rate sensor operates continuously or intermittently to give real-time data. Some premium devices also use ECG (electrocardiogram) sensors, which go a step further by detecting the electrical signals generated by the heart’s activity for more accurate heart rhythm tracking.
Accuracy
smartwatches equipped with a heart rate sensor tend to deliver highly accurate results when you’re not moving much.
During intense workouts: Less accurate due to motion — this is where the limitations of a wrist-based heart rate sensor become more noticeable, especially with sudden changes in pace.
Exercise Test Results
- Apple Watch: 186 bpm vs 188 bpm (true peak) — very accurate and showed excellent heart rate sensor responsiveness during sprints.
- Mi Band: 177 bpm — delayed tracking due to slower reaction from its sensor algorithm.
- Fitbit: Inaccurate during vigorous movement; struggled to stay on track during high-intensity activities.
Daily & Overnight Tracking
- Apple Watch: Super accurate during exercise, though it takes fewer readings, which may occasionally cause it to miss short spikes.
- Fitbit: Reasonably accurate and takes more frequent measurements, allowing its heart rate sensor to catch more variations.
- Mi Band: Least reliable overall, with a higher margin of error and slower response time.
Ideal Conditions (Overnight)
- All three devices performed well in stable environments like sleep, where there’s minimal motion and steady blood flow — ideal for heart rate sensor readings.
Average error:
- Apple & Fitbit: 1.8 bpm
- Mi Band: 3.4 bpm
Factors Affecting Accuracy
- Sudden changes in heart rate (e.g., sprinting).
- Motion artifacts (especially during weightlifting or cycling).
- Improper watch fit.
- Low skin temperature.
- Skin tone or tattoos, which may interfere with heart rate sensor light absorption and reflection.
- Obesity, which affects optical signal quality due to skin thickness and blood flow variability.
Gold Standard Comparison
While smartwatches have improved significantly, chest strap monitors remain the gold standard for workout accuracy. That said, wrist-based heart rate sensors offer far more comfort and convenience for all-day passive tracking and are becoming more accurate with each generation of smartwatches.
Step Counting
Smartwatches detect movement using an accelerometer combined with proprietary step-counting algorithms.
Real-World Performance
- Normal walking: Consistently accurate across Apple, Fitbit, and Mi Band.
- Holding something still: Apple & Mi Band correctly recorded zero steps.
- Silly walking: Mi Band overcounted significantly; Apple Watch was closest to the actual step count.
Algorithm Behavior
- Fitbit: Prioritizes not missing any steps — tends to overcount.
- Apple Watch: More conservative and prefers to skip ambiguous movements — may slightly undercount.
- Mi Band: Inconsistent — results depend heavily on your movement pattern.
Conclusion
Step tracking across these devices is generally solid and nearly as accurate as research-grade pedometers, though influenced by algorithm design and user behavior.
Blood Oxygen Level (SpO2)
How It Works
- Uses red and infrared light to calculate oxygen saturation.
- Oxygenated blood absorbs more infrared light.
- Deoxygenated blood absorbs more red light.
Accuracy
- Reliable for general wellness checks.
- Lab-tested to be within ~3% margin of error.
- Not dependable for emergencies like COVID-19.
- Most smartwatches lack FDA clearance for medical-grade SpO2 use.
Sleep Tracking
How It Works
Smartwatches use a heart rate sensor and motion detection to analyze sleep:
- Lower heart rate and stillness suggest you’re asleep.
- Algorithms classify sleep into light, deep, or REM stages.
Accuracy Metrics
- Total Sleep Time: ~10–30 minutes of deviation — steadily improving.
- Sleep Stages: Historically weak but getting better in newer models.
- Lying still while awake may be misinterpreted as sleep.
Note: Overnight conditions (dark, still) offer ideal settings for heart rate sensor accuracy.
Calorie Burn (Energy Expenditure)
Estimates Based On:
- Motion data from accelerometer
- Personal stats (age, height, weight, sex)
- Heart rate sensor (optional for improved accuracy)
- Skin temperature sensors (in some models)
Accuracy Issues
- Can underestimate calorie burn, especially for cycling or resistance training.
- Varies significantly across brands.
- Complex human physiology makes precise measurement difficult.
Blood Pressure Monitoring
Current Tech
- Omron HeartGuide: The only FDA-cleared smartwatch using an inflatable cuff.
- Other smartwatches use experimental PPG-based methods for estimating BP.
Best Practice: Still recommended to use standard upper-arm cuffs for accurate and trusted blood pressure readings.
Overall Usefulness and Limitations
Pros
- Promotes active lifestyle: average of +1800 steps/day.
- Encourages weight loss: modest but measurable.
- Tracks long-term patterns in sleep, activity, and heart rate.
- Can alert you to unusual trends (e.g. high resting heart rate).
Limitations
- Not medical-grade: not suitable for diagnosing stroke or heart attacks.
- Accuracy varies with skin tone, activity, and brand.
- May cause false alerts or anxiety.
- Privacy concerns: health data storage isn’t always secure.
Final Thoughts
Smartwatches are evolving into powerful health companions. While not a substitute for medical testing or diagnosis, they:
- Help you understand your body better
- Encourage consistent, healthy habits
- Offer week-to-week trends that are more reliable than memory
Always consult a healthcare professional for clinical insights and let them know about your smartwatch data if you’re tracking symptoms.
Other Clever Sensors
Smartwatches integrate even more sensors to offer a wide range of features:
- Skin Temperature Sensors: These measure thermal energy from your skin, tracking subtle changes over time that might indicate illness. Some even support women’s period tracking and fertility prediction. (Note: Some watches have “ambient temperature sensors” which measure the environment’s temperature, not your body’s).
- GPS (Global Positioning System): This uses satellite signals to pinpoint your exact location and track your movement, leading to more accurate step counts and the ability to map out your running or cycling routes. However, it can drain your battery faster.
- Microphone: Allows you to make calls directly from your watch, even if your phone is in your pocket.
- Barometric Pressure Sensor: Measures atmospheric pressure, which can be used to determine altitude.
- Magnetometer: Measures magnetic field strength and direction for navigation and orientation.
- Skin Conductance Sensor: Measures electrical conductivity of your skin, which changes with sweat levels, offering an indirect measure of stress and arousal.
The Big Question: How Accurate Are They?
Okay, so we know how they measure, but can we trust the numbers? The short answer is: it varies, and it’s getting better!
Step Counting: Mostly Good, But Watch Out!
Trackers are generally “pretty good” at counting steps, often almost as accurate as older, research-grade pedometers. However, specific activities and brands can make a difference:
| Scenario | Apple Watch | Fitbit | Mi Band 6 | Phone (in pocket) | Actual Steps | Insights | |
|---|---|---|---|---|---|---|---|
| Normal Walking | 469 | 475 (Exact) | 470 | 489 (worst) | 475 | Devices were generally close, phone overestimated the most. | |
| Carrying Glass (Hand Still) | 0 | Pretty close | 0 | Pretty close | 472 | Apple Watch and Mi Band counted zero due to hand stillness; Fitbit and phone were accurate. | |
| Silly Walking (Flailing Arms) | Nearly Right | Substantially Overcounted | Substantially Overcounted | Substantially Overcounted | Not provided | Apple Watch was impressive; others overcounted significantly. | |
| Driving (Sitting Still) | N/A | Overcounted ~1000 steps | N/A | N/A | 0 | Fitbit’s algorithm would rather not miss a step, prone to overcounting. |
This shows that different algorithms lead to different results. Fitbit, for example, is “renowned for over-counting” because its algorithm aims to not miss a step, while Apple’s is “fussier,” preferring to miss a few steps than count a false movement. The Mi Band, unfortunately, “somehow manages to be the worst of both worlds”. So, if your Fitbit-wearing friend always seems to out-pace you, don’t fret – it might just be the brand!
Heart Rate Tracking: Excellent at Rest, Tricky During Exercise
When it comes to heart rate, smartwatches equipped with a heart rate sensor are very good at measuring your heart rate when you’re resting, but less reliable during intense movement, where they tend to underestimate. The performance also varies quite a bit depending on the brand and how advanced the heart rate sensor technology is.
Controlled Experiment with a Gold Standard
In a test using the Polar H10 chest strap (a medical-grade reference):
During a Run
- Apple Watch: Started slightly high but closely followed the actual heart rate, even hitting 186 bpm when the actual peak was 188 bpm. The heart rate sensor in the Apple Watch was the most accurate in these tests.
- Fitbit: Consistent but noisier. During the final sprint, it completely lost accuracy—recording a much lower heart rate than expected. It’s known to be less reliable during high-motion workouts.
- Mi Band: Delivered the most delayed results. Its sensor took time to catch up and only peaked at 177 bpm, well below the actual value. Among the three, it was the least accurate—but also the most affordable.
All-Day and Overnight Tracking
Overnight, smartwatches shine. With less movement, darkness, and consistent conditions, the heart rate sensor performs at its best:
- Apple & Fitbit: Extremely close to chest strap readings with average errors of just 1.8 bpm.
- Mi Band: Slightly less accurate, with an average error of 3.4 bpm—but still acceptable given its price point.
Daytime Accuracy
- Fitbit: Provided fairly accurate data during the day and captured most spikes well—making it a solid all-rounder.
- Apple Watch: Typically super-accurate, but its heart rate sensor checks less frequently—around every 4 minutes—so it might miss short bursts in activity.
- Mi Band: Lowest performer again, with noticeable inaccuracies (e.g., reading over 90 or under 50 bpm when the actual rate was closer to 70 bpm). It only captures data once per minute.
Limitations of Optical Heart Rate Sensors
Optical heart rate sensors, while advanced, have some common challenges:
- Sudden spikes or drops in heart rate (like sprinting or interval training) are hard for sensors to keep up with.
- Sometimes, step movement can be misinterpreted as heartbeats—leading to incorrect readings.
- Motion artifacts during activities like weightlifting or cycling can distort the signal.
- Darker skin tones and tattoos can interfere with light absorption and reflection, making readings less accurate.
- Loose or overly tight straps reduce skin contact, which is crucial for accurate sensor function.
- Cold skin reduces blood flow, affecting signal detection.
- Obesity can alter signal strength due to changes in skin thickness and blood vessel patterns.
Blood Pressure: Still a Work in ProgressSmartwatch blood pressure tracking is still a developing technology. Currently, the Omron HeartGuide is the only smartwatch with FDA clearance, and it uses a traditional inflatable cuff mechanism for accuracy.
While some brands are exploring PPG-based methods (similar to those used by the heart rate sensor), they’re not yet reliable enough for clinical use. For now, standard upper-arm cuffs remain the best method for blood pressure monitoring.
Blood Oxygen (SpO2): Good for General Ideas
Smartwatches generally do “pretty well” at measuring blood oxygen levels when compared to pulse oximeters used in medical settings, with readings typically within about three percent. This can be helpful for people with lung conditions to monitor trends. However, most companies don’t have FDA clearance for their SpO2 features to be used as medical devices, and that small 3% difference could be misleading in critical situations like monitoring COVID-19 patients.
Calorie Burn: Take with a Grain of Salt
Trackers tend to be “pretty inaccurate” at calculating your calorie burn and often underestimate it. This is because energy expenditure is very complex and depends on factors like your body composition (muscle vs. fat), which most people don’t know. Exercises that don’t involve much wrist movement, like cycling, are particularly hard for them to track accurately. Some newer sensors are improving by adding heart rate sensor and heat sensing data.
Sleep Tracking: Time Asleep is Good, Stages… Not So Much
Smartwatches track your sleep by guessing based on your movements and heart rate.
- Total Time Asleep: They’re generally “quite good” at getting your total time asleep right, usually within half an hour. Recent Fitbits might overestimate by about ten minutes, which isn’t terrible.
- Sleep Stages (Light, Deep, REM): This is much harder. Historically, watches were “little better than guessing” when it came to classifying sleep into stages. They can’t tell if you’re dreaming! While newer algorithms from Fitbit and Apple are improving, smartwatches can still be fooled if you just lie still (they might think you’re repeatedly falling asleep and waking up when you’re actually just awake). One Apple Watch even thought its user was still in bed because it was plugged in to charge immediately after waking up!
Nighttime conditions are ideal for heart rate sensor readings—there’s little movement, and the dark environment limits signal interference, making sleep tracking more consistent.
The Bigger Picture: Expert Insights and Real-World Impact
While not perfect, these devices are continually evolving. Experts believe that smartwatches are “accurate enough to be useful.”
Here’s what the science says about their impact on your health:
- Early studies were not very encouraging regarding behavior changes, with some even showing people with trackers lost less weight—possibly rewarding themselves with food for being active.
- However, newer devices with more advanced heart rate sensors and improved usability are “more sticky” with better battery life, easier interfaces, and social features that help keep users engaged.
A large review from August 2023, combining 39 systematic reviews with almost 164,000 people, concluded that activity trackers do have “modest health effects”:
- Increase physical activity by an average of 1,800 steps per day or about 40 more minutes of walking.
- Lead to an average weight loss of about two pounds.
These benefits are seen across all age groups, including those with chronic conditions—some even experiencing improvements in blood pressure and cholesterol.
From a Doctor’s Perspective
Doctors at Johns Hopkins Digital Health Labs find smartwatches, phones, and heart rate sensor-equipped monitors helpful for people to stay in touch with their health.
The FDA has approved heart health tracking features on devices like the Apple Watch and Fitbit, including ECG functionality.
While smartwatches are not yet considered clinical-grade diagnostic tools, cardiologists may still recommend them for specific conditions—like tracking the frequency of atrial fibrillation (AFib) episodes to aid in medication adjustment.
That said, these devices are extremely limited in detecting serious events like heart attacks, heart failure, or strokes. If you experience chest pain or shortness of breath, always consult a healthcare professional.
The Future is Bright
Smartwatches are steadily transforming into health trackers, and many experts believe they’ll soon become regular tools in healthcare diagnostics and chronic disease management.
As heart rate sensor technology evolves and merges with AI, smartwatches will provide deeper and more actionable insights.
Future advancements may even bring smart sensors embedded into clothing, making separate wearables obsolete.