How does apple watch know sleep stages?

These smartwatches are good for general tracking, but they are not medical-grade devices. I had severe sleep apnea which my watch didn't clearly catch, and it was only diagnosed after a proper sleep study recommended by my doctor. Please consult a physician if you have chronic sleep issues.

Actually, it uses the green LED lights on the back to measure your heart rate and breathing patterns, yaar. When you combine that with how much your wrist moves, their algorithm guesses if you are in REM, core, or deep sleep. It is pretty smart but not 100% accurate.

I understand how checking these sleep stage graphs every morning can actually make you feel anxious if the deep sleep bar looks low. The watch just estimates stages based on motion and heart rate signals. Try not to worry too much about minor fluctuations; how you actually feel when you wake up is what matters most.

I totally get it, we all get fascinated by how these smart gadgets track our sleep cycles. It uses an accelerometer to detect your movements and a heart sensor to measure beat-to-beat changes. It is a neat way to get a rough idea of your rest, but don't stress too much over the daily numbers.