Doesn't sleep when youtube is left open

So I was wondering if anyone else had the problem that when youtube is left open, doesn't matter if it's just the homepage or a loaded video page, and the macbook just doesn't sleep. I would watch a video, leave it open and come back and it's on the black screen but it's not in sleep mode. Then the computer gets super hot because it's running at 3krpm idle and watching a video immediately makes the fans run higher. Any suggestions? Maybe flash player?