Many years ago I got to jumpseat in an AC 767 - this was before I started flying and it was great. At that time AC had a policy that every 20 minutes an FA was to enter the cockpit to make sure the pointy end as awake! Not sure if this meant a nap for one was OK.
To me it makes a lot of sense to have a reasonable policy with some guidelines and back. Even a quick nap can be very helpfull in restoring alertness.