Computer Will Not Go To Sleep When Playing Video

This is a problem that has only been occurring over the last month or 2 (had my computer for 2/3years). Quite often I will watch something on my computer when i go to bed - either a dvd, or streaming something from a website. Sometimes i fall asleep whilst these programmes are still playing and i will wake up in the morning to find my computer and monitor still turned on, instead of doing what they should do - monitor to turn off after 20minutes, computer to sleep after 45minutes.The computer will go to sleep after the desired time if its not doing anything at all so i assume it has something to do with the media settings/files. I have a custom plans setting for power options and in advanced settings > multimedia, it IS set to 'allow the computer to sleep when sharing media, so im not sure what else to try.As an additional note, this is something ive started to notice but im not so sure how relevant it is:The odd thing is, it seems to vary when it decides it wants to sleep. I was playing things through windows media player and it wouldnt sleep after 45minutes.. so i tried playing things through vlc media player instead. That seemed to work as the computer would go to sleep, so i figured it was just the programme causing the problem, but then after a few days of using vlc the computer wouldnt sleep anymore. The same goes for streaming stuff - using megavideo the computer wouldnt sleep, but when it was streamed from videobb it would. Then after a few days of using videobb it would stop going to sleep again...