What is the Sleep App monitoring to determine when you are slaaping

This does not tell me what the watch is tracking to determine if I am asleep or not. I understand it to be differences in my breathing which is interpreted by the watch as indicating that I am asleep or not. However I have work the watch two nights this week and both nights it shows I never slept for the 8+ hour I was in bed. The other functions work as far as monitoring my heart rate, activity level, etc. so I am wondering why the watch is showing I am not asleep when I am in bed and not moving. My doctor has seen the record of my sleep over the last several weeks and interprets it that I have sleep apnea and has ordered a sleep study. So again the question from me is what exactly is the watch monitoring to determine if I am asleep. If it is changes in my breathing pattern then so be it.