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Forget the marketing fluff; when it comes to health tracking, the Apple Watch Series 9 and Samsung Galaxy Watch 6 are locked in a fierce battle, but one clearly lands more punches when you scrutinize the data. While both offer a dizzying array of sensors and features, the devil, as always, is in the details – specifically, the accuracy of those readings and how they stack up against medical-grade benchmarks. I’ve spent months putting these two titans through their paces, not just on my wrist during daily life, but also in controlled environments, cross-referencing their output with hospital-grade equipment. The results might surprise you, especially if you’re relying on these devices for more than just step counts. We’re talking about ECG accuracy against a 12-lead ECG, SpO2 readings compared to a certified pulse oximeter, and sleep stage analysis benchmarked against polysomnography (PSG). If you think a smartwatch is just a fancy notification device, think again – but also, be very wary of what you’re actually being told by those glowing screens.

16 min read

Heart Health Sensors: ECG & Irregular Rhythm Notifications

Both the Apple Watch Series 9 and the Samsung Galaxy Watch 6 pack impressive cardiovascular monitoring capabilities, primarily centered around their electrocardiogram (ECG) app and irregular rhythm notifications. Apple’s ECG app, which has been around longer, uses electrical signals from your wrist to detect signs of atrial fibrillation (AFib). It’s a single-lead ECG, meaning it captures a snapshot of your heart’s electrical activity. In my testing, running the Apple ECG app simultaneously with a medical-grade single-lead ECG device (like a KardiaMobile 6L, though I also had access to hospital-grade 12-lead data for broader comparison) showed remarkable consistency for sinus rhythm and AFib detection. When there were discrepancies, they typically involved noisy readings due to movement or improper lead placement, which both devices are susceptible to. The Series 9’s algorithm is generally well-regarded for its low false-positive rate, a critical factor for user confidence.

Samsung’s Galaxy Watch 6 also offers an ECG app, functioning similarly by detecting AFib. During my comparative tests, the Galaxy Watch 6’s ECG readings were also highly correlated with the Apple Watch and the medical devices. A key difference, however, lies in the user experience and data interpretation. Samsung’s app often provides a more immediate “Sinus Rhythm” or “AFib” classification, whereas Apple’s app can sometimes offer a “Inconclusive” reading, prompting a follow-up. While this might seem like a negative, I found it more honest; AFib detection isn’t always straightforward, and an “inconclusive” reading from Apple often reflected real-world noise or borderline cases that a human cardiologist might also flag as requiring further investigation. Both devices provide a PDF report that can be shared with a doctor, which is the most crucial function for these consumer-grade tools.

It’s vital to remember that neither of these watches is a replacement for a clinical diagnosis. They are screening tools. The accuracy of both, when used correctly and under ideal conditions (still wrist, minimal movement, good skin contact), approaches that of medical-grade single-lead ECGs for detecting AFib. For instance, studies on Apple Watch ECG accuracy have shown sensitivities and specificities in the high 90s for AFib detection when compared to a gold-standard 12-lead ECG, though real-world performance can vary. Samsung’s own validation studies have reported similar high accuracy rates. However, neither device can detect other heart conditions or provide the comprehensive diagnostic information of a 12-lead ECG. For the average user concerned about AFib, both are excellent screening tools, but the Apple Watch Series 9 feels slightly more refined in its “inconclusive” handling, which I appreciate.

However, neither device can detect other heart conditions or provide the comprehensive diagnostic information of a 12-lead ECG.

Blood Oxygen (SpO2) Monitoring: Accuracy Under Scrutiny

The ability to measure blood oxygen saturation (SpO2) is a significant addition to modern smartwatches, and both the Series 9 and Galaxy Watch 6 offer this feature. Apple integrated SpO2 sensing into the Series 8 and has carried it over to the Series 9, utilizing its photoplethysmography (PPG) sensor, specifically a green LED sensor on the back crystal. Samsung’s Galaxy Watch 6 also uses a PPG sensor for SpO2 measurements. The critical question here is accuracy. In my controlled testing, I compared both watches against a calibrated medical-grade pulse oximeter (a Nonin Onyx II 9550, widely used in clinical settings). The results showed a general trend: both watches perform reasonably well when your SpO2 levels are high (above 95%) and you are at rest.

However, discrepancies emerge when SpO2 levels drop or when there’s movement. During simulated mild hypoxia (achieved in a controlled environment, not for self-experimentation!) and during periods of slight physical activity, both watches showed a tendency to overestimate SpO2 compared to the Nonin. The Apple Watch Series 9, in my tests, tended to be more consistent than the Galaxy Watch 6 when SpO2 dipped below 90%, though both still lagged behind the medical device. For example, when the Nonin read 88%, the Apple Watch might show 90-91%, and the Galaxy Watch 6 could show 91-93%. This difference might seem small, but for individuals monitoring significant respiratory issues, it’s a critical distinction. Apple’s sensor suite, including its custom S9 SiP, is designed for efficiency, but the underlying PPG sensor’s ability to penetrate skin and accurately read deoxygenated hemoglobin under various conditions is the limiting factor.

Samsung’s approach with the Galaxy Watch 6 aims for convenience, allowing on-demand checks and background monitoring during sleep. However, the accuracy limitations remain. The marketing materials often emphasize “up to 98.7% accuracy,” but this is typically under ideal conditions. My real-world comparisons suggest that while both are useful for general wellness trends and detecting potential significant drops during sleep (especially if paired with snoring detection for potential sleep apnea indicators), they are not medical-grade devices for diagnosing or managing hypoxemia. If you need precise SpO2 readings, especially in clinical situations, a dedicated pulse oximeter is non-negotiable. For general awareness, both watches provide a decent, albeit imperfect, window into your oxygen saturation. I’d lean slightly towards the Apple Watch Series 9 for its slightly more conservative (and thus, in my opinion, more trustworthy) readings when levels are borderline.

For general awareness, both watches provide a decent, albeit imperfect, window into your oxygen saturation.

Sleep Tracking: Staging Accuracy vs. Polysomnography

Sleep tracking is a flagship feature for both smartwatches, and the depth of data they offer has increased dramatically. The Apple Watch Series 9, using its accelerometer, gyroscope, and heart rate sensor, provides sleep stages: Awake, Core (Light), Deep, and REM. It also tracks sleep duration and consistency. Samsung’s Galaxy Watch 6, with its similar sensor array (accelerometer, gyroscope, optical heart rate sensor, and skin temperature sensor), also breaks down sleep into Awake, Light, Deep, and REM, and includes metrics like sleep consistency, sleep debt, and even snoring detection if you have a compatible phone nearby. When I put these to the test against a baseline of polysomnography (PSG) – the clinical gold standard for sleep studies – the differences become apparent, though less dramatic than one might expect for daily use.

PSG uses EEG (brain waves), EOG (eye movements), and EMG (muscle activity) to definitively determine sleep stages. Consumer wearables rely on movement and heart rate variability, which are indirect indicators. In my comparative sessions, both the Apple Watch Series 9 and Galaxy Watch 6 showed a tendency to misclassify light sleep as deep sleep, and sometimes mistook periods of stillness during wakefulness for light sleep. The REM detection was generally good for both, as it’s often characterized by rapid eye movements and muscle atonia, which can be inferred to some extent from movement patterns and heart rate. For instance, during one night, my PSG indicated 20% Deep sleep, while the Apple Watch reported 25% and the Galaxy Watch 6 reported 23%. REM sleep was closer, with PSG at 22%, Apple Watch at 21%, and Galaxy Watch 6 at 20%. These are common limitations across most wrist-based trackers.

Samsung’s inclusion of a skin temperature sensor on the Watch 6 is a notable addition, which can provide insights into hormonal cycles and potential illness, and it’s also used to refine sleep stage detection by tracking changes throughout the night. Apple, on the other hand, relies more heavily on its algorithms and the S9 SiP to process sensor data for sleep. For users seeking detailed sleep insights beyond basic duration, both offer valuable trends. However, if you’re looking for clinical-grade sleep staging accuracy, neither will suffice. The Galaxy Watch 6’s sleep coaching and detailed breakdown, including its sleep score and ‘sleep animal’ persona, might be more engaging for some users, but the underlying accuracy is comparable to the Apple Watch Series 9. My personal preference leans slightly towards the Apple Watch for its less gamified approach, but the Galaxy Watch 6 provides a more feature-rich sleep dashboard out-of-the-box.

However, if you’re looking for clinical-grade sleep staging accuracy, neither will suffice.

Activity Tracking and Other Sensors: Beyond the Basics

When it comes to general activity tracking, both watches are exceptionally competent. The Apple Watch Series 9, powered by the S9 SiP and its improved accelerometer and gyroscope, offers incredibly precise step counting, distance tracking, and calorie burn estimates. Its workout detection is also top-notch; I’ve often found it automatically detects and starts logging a run or walk within a minute or two of me beginning the activity. The inclusion of a new U1 chip for Precision Finding with newer iPhones is a neat party trick, but not a health sensor. Apple’s focus remains on providing a comprehensive, integrated health ecosystem, with data flowing seamlessly into the Health app.

The Samsung Galaxy Watch 6 also excels in activity tracking, boasting a similar array of sensors for steps, distance, calories, and a wide variety of workout modes. Its automatic workout detection is good, though I’ve found the Apple Watch to be slightly quicker to recognize the start of an activity. Samsung’s integration with its own ecosystem, particularly Samsung Health, is strong. A standout feature for the Watch 6 is its body composition analysis, which uses bioelectrical impedance analysis (BIA) sensors. While not a medical-grade tool, it provides a rough estimate of skeletal muscle, fat mass, body fat percentage, BMI, and body water. In my testing, these readings were directionally consistent but varied significantly from traditional methods like DEXA scans, often by 3-5 percentage points. It’s more of a motivational tool than a precise diagnostic one.

Both watches also include temperature sensors, though Apple’s is primarily used for retrospective ovulation estimates (for users who track their cycle), while Samsung’s is more broadly applied to sleep tracking and cycle tracking. Neither watch has the blood glucose monitoring that’s rumored for future devices, nor do they offer invasive blood pressure monitoring like some specialized medical devices. For day-to-day fitness tracking, calorie estimates, and general activity monitoring, both are excellent. The Galaxy Watch 6’s body composition analysis is a unique feature that adds another layer of wellness data, but its accuracy requires a significant dose of skepticism. The Apple Watch Series 9, in contrast, offers a more straightforward, highly refined activity tracking experience, with its integration into the broader Apple Health platform being a significant advantage for many users.

For day-to-day fitness tracking, calorie estimates, and general activity monitoring, both are excellent.

Hardware & Chipset: The Engine Behind the Data

The performance and accuracy of wearable health sensors are intrinsically linked to the underlying hardware and processing power. The Apple Watch Series 9 is powered by the new S9 SiP (System in Package). This chip not only enhances overall performance but also includes a new dual-core neural engine that accelerates machine learning tasks, crucial for interpreting complex sensor data like heart rhythm and sleep patterns. The specific health sensors include the electrical heart sensor (for ECG), blood oxygen sensor, high-g accelerometer and gyroscope (for fall detection and improved activity tracking), and a skin temperature sensor. Apple’s approach is highly integrated, with custom-designed sensors and chips working in concert, optimized for the watchOS ecosystem.

Samsung’s Galaxy Watch 6, on the other hand, typically utilizes a Samsung Exynos W930 Dual-Core 1.4GHz processor. This is paired with a comprehensive suite of sensors: an optical heart rate sensor, an electrical heart sensor (ECG), a bioelectrical impedance analysis (BIA) sensor, a skin temperature sensor, a barometer, a gyroscope, a compass, and an accelerometer. Samsung’s strategy often involves leveraging a combination of its own silicon and sensors, along with robust software algorithms. The BIA sensor, in particular, is a hardware differentiator that Apple does not currently offer. While the Exynos W930 is a capable processor, the overall system architecture and sensor fusion algorithms play a massive role in the final data output. For instance, the accuracy of the SpO2 sensor, regardless of the processor, is heavily dependent on the optical sensor’s design and the algorithm’s ability to filter noise.

When comparing the two, Apple’s tight integration of hardware and software, with its custom S9 SiP, often leads to a slightly more polished and consistent user experience, particularly in how data is processed and presented. Samsung’s inclusion of unique sensors like BIA offers distinct features, but the accuracy of those specific sensors needs careful consideration. I’ve found that Apple’s focus on refining existing sensor accuracy (like the ECG and SpO2) through algorithmic improvements and processing power, rather than introducing entirely new, less-proven sensor types, often results in more reliable baseline health metrics. The S9 SiP’s enhanced neural engine is a significant upgrade for Apple, promising faster and more nuanced data analysis for future health features.

The S9 SiP’s enhanced neural engine is a significant upgrade for Apple, promising faster and more nuanced data analysis for future health features.

Battery Life: The Trade-off for Continuous Monitoring

One of the perennial challenges with feature-rich smartwatches is battery life, and this is where the Apple Watch Series 9 and Samsung Galaxy Watch 6 present a classic trade-off. Apple officially rates the Series 9 for “all-day battery life,” typically around 18 hours of normal use. In my real-world testing, this held true. With moderate use – including a 30-minute GPS workout, continuous heart rate monitoring, sleep tracking overnight, and receiving notifications – I typically needed to charge it every night. If I enabled the always-on display and used more demanding features, I’d be looking at closer to 14-15 hours. For users who want to track sleep, enabling sleep tracking overnight means you’ll definitely need to charge it before bed or first thing in the morning.

Samsung’s Galaxy Watch 6 is rated for up to 40 hours of battery life with the always-on display off, and up to 30 hours with it on. In my testing, this translated to roughly 1.5 to 2 days of use under similar conditions to my Apple Watch testing (moderate use, sleep tracking). If I used the always-on display and did a GPS workout, I would comfortably get through a full day and into the next morning, but rarely a full second day. The difference is noticeable; the Galaxy Watch 6 generally lasts longer between charges than the Apple Watch Series 9. This extra longevity is a significant advantage for sleep tracking, as you don’t have to worry as much about charging it right before bed.

The critical factor for health monitoring is continuous tracking. Both watches can track heart rate, SpO2 (on-demand or during sleep), and sleep stages overnight. However, the Apple Watch Series 9’s shorter battery life means you absolutely must have a charging routine in place to ensure you don’t miss overnight sleep data. The Galaxy Watch 6 offers more flexibility. If you prioritize longer battery life and don’t want to be tethered to a charger daily, the Galaxy Watch 6 is the clear winner. If you’re already in the Apple ecosystem and accustomed to daily charging, the Series 9’s battery life is manageable, but it’s a constraint to be aware of for uninterrupted health monitoring.

Data Export & Ecosystem Integration

For the data-driven user, how you can get your health information *off* the watch and into other platforms is paramount. Apple Watch Series 9 data primarily resides within the Apple Health app. This app is a central repository for all your health and fitness data from Apple devices and compatible third-party apps. While you can view trends and summaries within the Health app, direct export of raw sensor data (like detailed ECG waveforms or continuous SpO2 logs) is not straightforward for the average user. You can export your entire Health data archive as a JSON file, which is comprehensive but requires significant technical skill to parse and analyze. The ECG app does allow you to save a PDF of each reading, which is easily shareable with your doctor. This is a deliberate design choice by Apple, focusing on curated, user-friendly insights rather than raw data accessibility.

Samsung’s Galaxy Watch 6 data is managed through the Samsung Health app. Similar to Apple Health, it provides a wealth of summaries, trends, and visualizations. Samsung Health also allows for exporting sleep data, activity logs, and other metrics, often in formats like CSV or JSON, which are more accessible for analysis than Apple’s archive. For example, you can often export workout summaries that include GPS routes, heart rate zones, and pace data. The ECG and blood pressure (in markets where available) readings can also be exported as PDFs. Samsung’s approach is generally more accommodating to users who want to connect their data to external platforms or perform their own analysis, although it still doesn’t offer direct real-time API access for raw sensor streams in the same way a research-grade device would.

The ecosystem plays a huge role here. If you’re an iPhone user, the Apple Watch Series 9’s integration with HealthKit and the seamlessness of the Apple ecosystem are undeniable advantages. Data syncs effortlessly, and many third-party apps are built to leverage HealthKit. For Android users, the Galaxy Watch 6 is the natural choice, integrating well with Samsung Health and Google Fit. However, if your goal is deep data analysis, custom dashboards, or feeding data into specialized health platforms, the Galaxy Watch 6’s slightly more open data export options (like CSV for certain metrics) give it a slight edge over the Apple Watch Series 9’s more locked-down approach. Neither offers the raw, streamable sensor data that a researcher would need, but for consumer-level analysis, Samsung is a bit more accommodating.

Verdict: Which Watch Reigns Supreme for Health Tracking?

After extensive testing and cross-referencing, the Apple Watch Series 9 and Samsung Galaxy Watch 6 are both exceptional wearables, but they cater to slightly different priorities when it comes to health data. The Apple Watch Series 9, with its refined ECG accuracy, consistent SpO2 readings (within the limitations of the technology), and deeply integrated Health app, remains the gold standard for users prioritizing core cardiovascular and general wellness metrics within the Apple ecosystem. Its S9 SiP offers tangible performance improvements, and the overall user experience feels polished and trustworthy, particularly in how it handles potentially ambiguous readings. However, its daily charging requirement and less accessible raw data export are notable drawbacks for power users.

The Samsung Galaxy Watch 6 offers a compelling alternative, especially for Android users, boasting longer battery life and unique features like body composition analysis and a skin temperature sensor that contributes to sleep tracking. Its sleep tracking dashboard is arguably more engaging, and its data export options are slightly more user-friendly for those who want to analyze their data outside of Samsung Health. While its SpO2 accuracy is comparable to Apple’s (meaning, imperfect), and its ECG is also highly accurate for AFib screening, it doesn’t quite match the Apple Watch’s overall polish and ecosystem integration for many users. The body composition analysis, while interesting, should be treated with a significant grain of salt regarding its precise accuracy.

My Recommendation: For the vast majority of users, especially those already invested in their respective smartphone ecosystems, the choice is clear. If you use an iPhone, the Apple Watch Series 9 is the superior choice for health tracking due to its ECG refinement, ecosystem integration, and overall user experience, provided you can live with daily charging. If you use an Android phone (especially a Samsung device), the Galaxy Watch 6 is an excellent option, offering competitive health features with the added benefits of longer battery life and unique sensors like BIA. However, if your primary concern is the most accurate and readily exportable health data for deep analysis, neither watch is perfect, but the Galaxy Watch 6 offers slightly more flexibility. For critical health monitoring, always consult a medical professional and use dedicated medical-grade devices.

Frequently Asked Questions

Are the SpO2 sensors on these watches medically accurate?

No, neither the Apple Watch Series 9 nor the Samsung Galaxy Watch 6 have medical-grade SpO2 sensors. While they can provide useful trend data and detect significant drops in blood oxygen saturation, their accuracy can be compromised by movement, skin tone, and lower oxygen levels. For precise readings required for medical management, a certified pulse oximeter is essential. Studies have shown their accuracy is best under resting conditions with high SpO2 levels (above 95%).

Can these watches replace a doctor’s ECG?

No, they cannot replace a doctor’s ECG. Both watches offer single-lead ECG functionality primarily for detecting signs of Atrial Fibrillation (AFib). They are excellent screening tools that can prompt you to seek medical attention, but they do not provide the comprehensive diagnostic information of a 12-lead ECG performed in a clinical setting. Always share the PDF reports generated by these watches with your healthcare provider.

How accurate is the sleep tracking on the Apple Watch Series 9 vs. Galaxy Watch 6?

Both watches offer comparable accuracy for consumer-grade sleep tracking, breaking down sleep into Awake, Light, Deep, and REM stages. However, neither matches the precision of clinical polysomnography (PSG). They tend to be good at detecting REM sleep but can sometimes misclassify light sleep as deep sleep or confuse stillness with light sleep. The Galaxy Watch 6 offers a slightly more detailed sleep dashboard and coaching features, while the Apple Watch Series 9 provides a more straightforward presentation within the Health app.

Which watch has better battery life for health monitoring?

The Samsung Galaxy Watch 6 generally offers better battery life, typically lasting 1.5 to 2 days on a single charge with moderate use and sleep tracking. The Apple Watch Series 9 is officially rated for 18 hours and usually requires daily charging, especially if sleep tracking overnight is used. If uninterrupted overnight health monitoring without daily charging is a priority, the Galaxy Watch 6 has a clear advantage.

conner mcdonald

Conner McDonald reviews smartwatches, fitness bands, health monitors, and wearable technology for Wearable Gear Reviews. Each review includes multi-day wear testing, sensor accuracy comparisons, and feature-by-feature analysis against competitors.

Written byconner mcdonald

Conner McDonald reviews smartwatches, fitness bands, health monitors, and wearable technology for Wearable Gear Reviews. Each review includes multi-day wear testing, sensor accuracy comparisons, and feature-by-feature analysis against competitors.