Another problem with this sort of project is reliably detecting the human, a PIR will only detect movement, if they sit still for 5 minutes it will think they have left the room.
I have thought quite a bit about this case, but can't think of a way around it using a standard PIR. You can get microwave occupancy sensors but I guess they also work on the principles of changing values.
I think if I go ahead I will have the lights fade out when the time has elapsed, then if the room is occupied it gives the occupier a chance to keep it on!
haha, many hands... I like it