Sleep Detector Power Saver

I remember umpteen instances when I've seen people leave their lights and/or televisions turned on and fall asleep. While the person is recharging himself his appliances are continuously hogging power all night for no useful purpose at all. Of course the easiest solution to the problem is for people to realize the mistake and be responsible enough to turn off all unwanted utilities when they go to sleep.

But since we still have so many careless people around, there is a potentially huge market for a product that could detect sleep in a person and take action accordingly - switch off lights, TV etc. Sleep detection could be done based on monitoring of one or more of various parameters - heart rate or pulse rate, body movement, body temperature, respiration rate etc. The major challenge in such a product would probably be to identify which among these myriad options would provide the cheapest and the most reliable way to detect sleep. Any answers?

[This post is an elaboration of Problem-3 on the post "Nine Problems Worth Solving"]