I wish there was a device I could wear, maybe like a wristwatch, which could track whether I was awake (which is probably possible based on pulse and/or movement). It should tell my phone my status, and my phone would automatically go into silent mode while I'm asleep. It might as well display on Facebook and GChat too.

If it was accurate enough, the data on my (and everyone else's, if they chose to make it public) sleep patterns could be very interesting and even medically useful.