Yes, there's an actual website that promises to tell you the sleeping schedule of your Twitter friends. You can try it now at http://www.sleepingtime.org/.
Their algorithm "will determine the approximate sleeping schedule based on the time when he or she stays least active on Twitter".
Naturally, I put in my Twitter username to see when it thinks I sleep.
Not bad... I'm more of an 11pm to 7:30am guy, but midnight to 6 are definitely my core sleeping hours.
