Providing it is done properly - it is no threat to safety!
Well... short of requesting an alarm call from ops....
I was tempted many times to agree to a suggestion from an F/O that maybe we could take turns having a 15 min nap but as we were usually both in the same state I could never be sure that the one staying awake would do so..
Some times it got so bad I'd pull out the POH and start a study session on the systems..
