We have all driven a car beyond our ability to stay awake and the same thing happens in the cockpit. In a perfect world you would go to work and fly fully refreshed and ready for a 12 hr duty day but some of the time that doesn't happen. It is much safer to take a nap than tough it out and try to stay awake. Personally I flew planes that flew during the day but when forced to fly all nighters when junior called in sick a lot. I just could not be alert on all nighters and that is not safe. I know it is unfair for the reserve guy. I don't know how the all nighter guys do it.