try this

what you need is a few mostion sensors and a few wall infered light beams to bounce to each wall to measuer the distence and when no one is in the room the power will decreace till someone goes in to the room

spellchequer!!!!!!!!!!!!!!!!!!!

RVZ717 has the right direction. The market for occupancy controlled lighting has been around for several years and offers a pretty extensive selection of control options.

Here is just one example of a specific manufacturers offerings...

occupancy sensors for lighting

You could use a motion detector no motion then turn the power off. Or a light beam interrupt sensor that detects someone exiting the room. The cheapest is a heat sensor that you can buy and it detects motion but it may not work if a person is sleeping in the room (no motion).