How does Apple Watch Series 5 track sleep accurately?

Dr. Wants me to track my sleep. If I have sleep focus on my phone does not play the audible alarm for my diabetes sensor (my sugar tends to drop at night), it only vibrates on the watch (this does not always wake me which can be dangerous). If I disconnect the wifi and bluetooth on the watch, will it still track my sleep? Not really sure how well it tracks it anyway because I could be reading (or tv) and the watch thinks I am sleeping 🤷‍♀️