I've been using the Oura ring for that and it works pretty well for me. Unlike the other fitness trackers, its primary focus is sleep. I don't know how well it would handle reading in bed, however.
If you buy directly from Fitbit's website, the satisfaction guarantee is 45 days so you have plenty of time to try it out and see how well it works for you.
It also easily tells between activities in bed (even motionless ones like reading or watching) vs sleeping (and all the assorted cycles). It also measures heart rate, breathing (and how many times you stop), and even snoring.