Sensor/Set of sensors to detect particular human state

I want to be able to use some sensors to be able to find out if the human wearer of that sensor is awake or asleep.

It can be one sensor or many, but they need to be fairly non-invasive i.e. not requiring the user to wear a huge piece of machinery on their head. I hear the human body temperature changes when awake/asleep so perhaps a temperature sensor would be a good one, but surely there will be more things? I know the breathing rate also changes from awake to asleep so maybe this can be measured from afar somehow (without putting something in the person's mouth or under it).

Please share if anyone thinks of any others.

Maybe heartbeat?

This will be tremendously difficult. Brain waves are not a reliable indicator, as the brainwaves when dreaming look just like when you are awake. Closed eyes, no. Breathing, I don't see it as reliable as just like brain waves, it changes during dreaming.

It might help if you tell us what you are trying to accomplish.