Napping
There have been many studies on napping, started in the late 80's, and is pretty much allowed in cruise with many operators, both airline and corporate, based on having a crew as sharp as possible on the approach. I believe Nasa spent a considerable amount of time studying what worked best and a figure came out that up to 25 minutes was acceptable, any more and it was difficult to keep sharp. Most of your long haul and or corporate long haul crews will nap at cruise when the need arises. But I do not think it was written down, although I do believe their are some airlines that have stated it is acceptable in their long haul flights, but I cannot bring to mind where they have stated it.
With the long haul capabilities of newer aircraft, sometimes with minimum crew, you need the crew sharp for the approach, as opposed to cruise when it is quiet, like on a over water crossing where one crew memeber takes over control of the aircraft, whilst the other shuts his eyes, turns off the headset.