Agree with 411a in this case.
It is better to have a crew member take 10 or 30 for that matter, than 2 or 3 flight deck crew members all battling to stay awake for 8 to 10 hours at a time. All trying to stay awake in the above scenario increases the chances of "all crew nodding off" at once. Seen it happen myself, as battling to stay awake, is a source of fatigue, in itself.
At least when a crew member has woken up from a nap, you have at least one crew member relatively "fresh" and this has to be a great asset should there be an emergency.