How does apple watch know when you sleep?

It simply detects when your wrist stops moving for a long time and your heart rate drops to a resting state. It is a very convenient feature to help you keep a general track of your rest.

It is quite simple actually. The watch has motion sensors that detect you are completely still, and the heart rate sensor checks if your pulse has slowed down to your usual sleeping rate. If both match, it marks you as asleep. No magic, just basic sensors.

It works through a combination of the built-in accelerometer tracking your movement and the photoplethysmography (PPG) sensor monitoring your heart rate. When your movement stops and your heart rate stabilizes, the algorithm logs it as sleep. It is highly reliable for daily tracking.

These modern watches use sensors to track your body's rest. While it is good to know, nothing beats the traditional habit of sleeping early and waking up with the sun. Drinking warm milk before bed also helps you sleep naturally without needing a watch to tell you.

Sometimes my watch logs me as sleeping when I am just lying very still on the couch watching TV! Does this happen to anyone else? It makes me doubt if the overall sleep duration it shows is actually correct.