Why does the VivoMove often only start recording sleep at midnight?

I've had this for 7 nights now. Worn the watch each night and configured start and and sleep time. It has now recorded only one full night of sleep. Missed one night entirely, another night the watch said two hours sleep - but it failed to synch to the app. And the other nights it recorded between 2-5 elapsed sleep hours only - even though I was clearly asleep in bed.