I installed SweetDreams and yes, it's a bit basic (I'd love to be able to customize how the applet looks and the 'sleep times', etc) - and I just tried it out for what I personally would use it for: listening to a podcast using Panucci at night whilst almost falling asleep.

I set it for '10 minutes' and started up my podcast and Panucci and what happened was that when the time is up the device goes into 'offline' mode and locks the keyboard. It didn't silence/stop Panucci as that continued to play though.