I never have this problem with screensavers (since I don't use them), but I do have it a lot with laptops falling asleep in the middle of a video, whether it's streaming from Youtube or playing locally in MPC or whatever. So if anyone's trying to implement this please make sure it prevents sleep mode as well as screensavers (I don't know if the OS treats them differently).
Incidentally, since the main use case seems to be video playing, maybe this would go better in sfeMovie than "core" SFML?