[coredog64]

There are two data points you need to accurately track sleep: Motion and respiration rate. With that information you can identify light sleep vs. deep sleep, and sometimes you can differentiate between light sleep and being still.

Source: Worked on the ResMed S+ and got a theory dump from the PhDs running the project.

[mandevil]

Sleep versus awake from a sensor on the wrist is always going to be subject to wide error bars.

Source: worked on an experiment trying to make people fall asleep while driving (in a simulated environment so it was safe- I built the simulated environment). It took a lot of EEG wires and gel in subject's hair etc. to accurately figure that out.

[draven]

I would think that "pressing the button to turn on the backlight so I can check the time" would be a good indication that I'm awake :)