I thought this sort of thing happened pretty often across all airlines, but was generally kept quiet.
Human pilots are only really needed at take-off, landing and in case of emergency. Once a plane is in the air, flight paths are pretty much fixed by the aviation authorities and the autopilot does basically everything. The pilot and co-pilot don't have much to do, get bored and doze off.
They're never supposed to be asleep at the same time, of course, but I would not be surprised if the other accidentally drifts off when the first is already taking a more official nap, especially on long haul flights.