Suddenly my Apple Watch begun to drain the entire battery in less than a day. Under normal usage at night, the lasting battery was always around 40%, but for no reason the watch started to drain the entire battery before 10 hours of usage. I unpaired and restore the watch backup but the problem persisted, the solution was to unpair and NOT restoring the watch backup, so a new installation solved the problem. I don't have any third party app installed on the watch, so I assume it is a Watch OS bug. Reported the bug on Apple's website and hopefully a fix will be available someday. I just wished I could troubleshoot the problem and figure out what is causing it.