My experience with Garmin with regards to sleep tracking is unfortunately not much better. It does measure the time I'm in bed, but if it takes me a while to fall asleep, or keep laying in bed after waking up, it will count that as sleep. I've even had a few times where it didn't notice that I woke up at night, went to the bathroom and then back bed. It would just count that as sleeping.

Since it can't even see the difference between being awake and sleeping, I absolutely don't trust anything it says about the amount of time I spend in each sleep phase.