Indeed! I am having the same issue- most days it says l slept only around 2-30am or 3am, whereas I go to bed between 11 and 11-30pm. I have a Withings Sleep device as well which does a better job, for sleep start time.
Also, I get up for a bathroom visit around 2-30, I think it restarts the sleep time after that.
Wearing the watch tightly does help sometimes, but not always.
I have tried resetting the watch several times, also have the latest software update.
Any tips appreciated.