How Apple Watch Tracks Your Heart, Steps and Sleep

How does your Apple Watch count your steps, read your heart rate and know when you're in deep sleep? It never sees your feet. Everything comes from a motion sensor, flashing green lights and a lot of maths, and one of those numbers is far less accurate than the others. Here's how Apple Watch tracking really works: - Steps: a motion sensor counts the rhythm of your arm swing, which is why pushing a trolley can cost you steps - Heart rate: green lights flash hundreds of times a second, blood soaks up green light, and tiny sensors catch the flicker of every heartbeat - Why a loose strap or some tattoos can throw the reading off - Sleep: Apple works out your sleep stages from the motion sensor every 30 seconds, using your movement and your breathing - Calories: nothing on your wrist can measure them, so the watch makes a guess. A 2025 review of 56 studies found heart rate off by about 4%, steps by about 8% and calories by almost 28% - The Apple Heart Study: over 400,000 people, and how the same green light can flag an irregular pulse - And WHOOP, the band with no screen that runs on the same green light Next time you check your watch, you'll know: it felt some movement, caught some light and did the maths. Trust the heart rate. Take the calories with a pinch of salt. Sources: Apple, University of Mississippi review (2025), Apple Heart Study (New England Journal of Medicine, 2019). Follow Binaryhood for more on how the tech you use every day actually works. #AppleWatch #FitnessTracker #Shorts #Tech