Originally Posted by Swanhunter

They may have had an early start for a connecting flight? Or indeed an overnight from N America.

Maybe a late one the previous evening and need to catch up on sleep? Or possibly an onward overnight connection on a non-flat bed airline and they'd like some nap.

Or possibly bored rigid by daylights and unenthusiastic about the Club food and service sleeping is the only way to get through the flight?