The only time I tried to sleep in ORD was during the day in concourse C with the dreaded chairs with arm rests.

I have since realized that not all benches in ORD have arms. Concourse F would have been a better place.

ORD is loud though. I had to wait until around 2:30 am once to depart and after midnight they start the "you must have a boarding pass and security may ask to see it or kick you out" announcements.