In sleep mode, watch says I'm awake when I'm asleep

My watch has tracked my sleep well, until the last 4 days. Suddenly, it's telling me I am awake for an hour and a half when I've been fast asleep. This morning, my alarm woke me from dreaming at 7am and yet when I looked at my stats, it said I'd been awake since 5.30.

This has happened for the last four days. My watch is not too loose and I am not moving in bed - I have a weighted blanket and it's impossible to thrash about under that!

What is going on? Why is it suddenly going 'rogue' like this?