Best technological way of detecting human going from awake to asleep

[asif49]

What is the best way, using technology, to discern whether someone has gone from an awake state to a sleep state. Ideally, this would be something passive and non invasive/not limited to the room or location the person is in.

Ideally this thing would be:
1) efficient
2) cheap
3) easy to acquire
4) utilising the sensors of an existing hardware device, like a mobile phone
5) if it's an existing hardware device, it should have an API that can be used for developing/doing something useful with the awake/asleep data
6) if it's not an existing hardware device, and rather a novel method you've been able to think of, then it should be easily programmable. I don't want to have to import moonrock for one of it's inner components or something.

Think out of the box!

[Agilemind]

The only way I know to be sure a person is asleep involves sensing brainwaves via electrodes on their head. You could probably get a decent proxy using some combination of motion/muscle tension, heart rate, respiration rate. Some stages of sleep can be detected using video of the person sleeping (eg. eye movements associated with REM sleep) but not the precise time of the transition from sleep to awake.

I was thinking brainwaves too (more theta / delta waves) - but these do require electrodes. Been a while since I was involved with this stuff, but I remember measuring saccadic eye movements and we had to scrape off some skin to make a good contact with the electrodes (with conducting gel). Not sure if things have moved on in 25 years.