*** For the latest v5 with integrated YouTube Web (controllable with the YouTube app on your phone) please download from https://www.microsoft.com/en-gb/p/sleep/9ppgrg6t7dcj ***
Do you use your computer for playback of films, streamed TV etc. before going to sleep?
In these cases, Windows' sleep functionality is suspended and won't kick in once the video has finished.
The solution for many is to use a sleep timer to send the computer to sleep after the estimated amount of time...