Watch 7 went from 77% to 0 overnight.

I charged my watch to about enough that it could track my sleep. It was at 77% and I fell asleep around 9. I woke up at 7 and it was dead?? The last of my sleep it tracked was at 4am. I don't understand why my watch could die that fast? I thought, maybe an update? But my watch is telling me that an update is available, not that it updated during my sleep.