• It doesn't need to say "sleeping", just "away", "busy" or "offline" like on most messaging services now and since forever.
• Sleep/availability detection is already automatic and can be improved.
• Extra details don't need to be public, only shared with [specific] friends. Unexpected calls from unknown contacts wouldn't be welcome anyway.
Plus, how on earth is constantly updating your status more convenient than changing the clock twice a year? Who is actually going to bother to do that? What problem does this solve exactly?