A smartwatch tracks your health, keeps you connected to your phone, and handles everyday tasks like payments and alarms, all from your wrist. Modern smartwatches pack heart rate sensors, GPS, accelerometers, and wireless connectivity into a device that also tells time. But the range of what they do has expanded well beyond step counting, so here’s a full breakdown.
Heart Rate, Blood Oxygen, and ECG
The most important sensor in a smartwatch is the optical heart rate monitor, which uses a technology called photoplethysmography. A small light (usually green or infrared) shines into your skin, and a detector reads how much light bounces back. Because blood absorbs light differently as it pulses through your vessels, the watch can calculate your heart rate continuously throughout the day, during workouts, and while you sleep.
The same basic sensor technology measures blood oxygen levels. Infrared light penetrates deeper into tissue and can distinguish between oxygenated and deoxygenated blood, giving you a percentage reading (typically 95% to 100% is normal). Many watches check this overnight to flag potential breathing disruptions during sleep.
Some watches also include electrocardiogram capability. A traditional ECG in a hospital requires at least three electrodes placed on different parts of the body. Smartwatches simplify this into a 30-second, single-lead recording: you place your finger on a contact point on the watch, completing a circuit through your chest. The FDA has cleared ECG features on four wrist-worn devices so far, including Apple Watch (Series 4 and later), Fitbit Sense 2, Samsung Galaxy Watch3, and Withings ScanWatch. These can detect atrial fibrillation, a common irregular heart rhythm, and send you an alert if something looks off.
Fitness and Activity Tracking
Step counting is just the starting point. Smartwatches use accelerometers and gyroscopes to detect movement patterns, and built-in GPS to map outdoor runs, bike rides, hikes, and walks with precise distance and elevation data. Most watches automatically detect common exercises and log them without you pressing a button.
Higher-end fitness watches estimate your VO2 max, which is a single number representing your cardiorespiratory fitness. It measures the maximum amount of oxygen your body can use during intense exercise. Your watch calculates this by combining heart rate data with GPS pace information during a run or brisk walk. Over weeks and months, watching this number trend upward is one of the most reliable signs that your cardiovascular fitness is improving.
Sleep Monitoring
Wearing a smartwatch to bed gives you a nightly breakdown of your sleep stages: light sleep, deep sleep, REM sleep, and time spent awake. The watch figures this out by combining movement data from its accelerometer with heart rate variability readings from the optical sensor. Heart rate variability shifts in distinct patterns during deep sleep as your nervous system stabilizes, which is why wrist-worn trackers are particularly good at identifying deep sleep stages.
That said, accuracy varies. A 2023 validation study tested 11 consumer sleep trackers against polysomnography (the gold-standard clinical sleep test). Wearable watches performed best at detecting deep sleep, with the Google Pixel Watch scoring highest in that category. REM and wake detection were less reliable across all devices tested. Sleep tracking on a smartwatch is useful for spotting trends and patterns over time, but treat the exact minute counts for each stage as estimates rather than precise measurements.
Notifications, Calls, and Connectivity
A smartwatch mirrors your phone’s notifications to your wrist. Texts, emails, calendar alerts, and app notifications all appear as they arrive, and you can read, dismiss, or reply to many of them without pulling out your phone. Most watches let you send short replies using voice dictation, preset responses, or a tiny keyboard.
Bluetooth-only watches need your phone nearby to relay notifications and handle calls. Cellular (LTE) models add an independent data connection, so you can leave your phone behind entirely and still receive calls, texts, and stream music. The tradeoff is battery life: LTE drains significantly more power than Bluetooth. In practice, most people keep Bluetooth connected when their phone is nearby, and the watch only switches to cellular when the Bluetooth connection drops. This makes LTE most useful for specific situations like running without your phone, going to the beach, or working in areas where phones aren’t allowed.
Contactless Payments
Most smartwatches have an NFC chip (near-field communication) that lets you pay at any contactless terminal by holding your watch face near the reader. Google Wallet, Apple Pay, and Samsung Pay all work this way. You add a credit or debit card through the watch’s companion app, and the card is stored securely on the watch itself. When a payment goes through, you’ll feel a vibration and see a confirmation on screen. It works at grocery stores, transit systems, coffee shops, and anywhere else that accepts contactless payments.
Voice Assistants and Smart Home Control
Smartwatches support voice assistants like Siri, Google Assistant, Alexa, and Bixby depending on the platform. You can launch them by tapping an icon or saying a wake phrase. From your wrist, you can set timers, create reminders, check the weather, send messages, and control smart home devices. If your lights, thermostat, or locks are connected to a smart home platform, you can adjust them by speaking to your watch.
Safety and Emergency Features
Fall detection is one of the most consequential smartwatch features. When the watch detects a hard fall, it taps your wrist, sounds an alarm, and displays an alert asking if you need help. If you’re still moving, it waits for your response. If you’ve been immobile for about a minute, it starts a 30-second countdown with an increasingly loud alarm. When the countdown ends without a response, the watch automatically calls emergency services, plays a pre-recorded message describing the situation, and shares your GPS coordinates. It then texts your emergency contacts with your location.
Apple Watch models with newer iPhones can even route emergency calls through satellite when there’s no cellular or Wi-Fi coverage. Emergency SOS features also let you manually trigger a call to local emergency services by pressing and holding the side button, which is useful in situations where you can’t reach your phone.
Apps and Productivity
Smartwatches run their own app ecosystems. Calendar apps like Google Calendar and Apple Calendar sync to your wrist, so you can glance at your next meeting without touching your phone. Task managers like Todoist and Things let you check off to-do items. Music apps let you store playlists offline or stream through a cellular connection, which is especially handy during workouts with wireless earbuds and no phone. Navigation apps provide turn-by-turn directions through haptic taps on your wrist, so you don’t need to stare at a screen while walking or cycling.
Battery Life Expectations
Battery life is the main constraint, and it varies enormously by device. A standard Apple Watch lasts about 18 hours on a single charge, meaning daily charging is non-negotiable. Samsung Galaxy and Google Pixel watches do better at roughly 40 to 48 hours. The Apple Watch Ultra 2 stretches to 36 hours in normal mode and 72 hours in power saver mode.
If battery life is your top priority, specialized fitness watches are in a completely different league. The Garmin Enduro 3 lasts up to 36 days per charge in daily use, or up to 90 days with regular solar recharging. Even its GPS tracking mode runs for 120 hours. The Garmin Instinct 3 with a solar-charging display can theoretically run indefinitely in basic mode with enough sunlight. The tradeoff is that these watches use simpler, lower-power displays and fewer “smart” features compared to Apple or Samsung models.
For most people choosing between a full-featured smartwatch and a fitness-focused one, the decision comes down to whether you want a mini smartphone on your wrist that needs nightly charging, or a rugged fitness tracker that lasts weeks but does less with notifications and apps.