Time awake — Those who have been continually awake for a long period of time since their last major sleep period are more likely to be fatigued.
Cumulative sleep debt — For the average person, cumulative sleep debt is the difference between the amount of sleep received over the past several days, and the amount that would have realized with 8 hr. of sleep a night.
Time on task — The longer a person has continuously been doing a job without a break, the more likely fatigue will result.
Individual variation — Individuals respond to fatigue factors differently and may become fatigued at different times, and to different degrees of severity.
All crew rest and duty time limitations have one ultimate goal, ensuring pilots are physiologically ready to accomplish the complex technical and analytical skills required to achieve their flight mission. There have been times, I'm sure, when any of us would have surrendered our thrones for a good night's rest.
Gulfstream 650s, Global Expresses, and Dassault Falcon 7Xs comprise magnificent evidence that the corporate world intends to routinely fly some very long stage lengths over multiple routes on multi-day missions. With airplanes that can fly from 12-16 hr. non-stop, pilots must not only have to try to launch rested, but stay rested while the plane is en route.
The hardest of all the factors to plan for is individual variations. I have flown trips where I crossed nine time zones and slept like a baby, then travelled three more and again slept the full night. I would add that most of these trips were in a Westward direction. I have flown other trips where I do not believe I had more than horizontal-but-sleepless rest for days on end.
What I regard as a major flaw in FAA's new rules is that they require crewmembers to report when they do not feel rested at the start of an FDP. That won't happen. But if it does happen, we will see a major disruption of scheduled and supplemental overseas flying.
The simple truth is that no one ever feels ready all of the time. I remember, during Operation Desert Shield/Storm looking for a young loadmaster who had failed to report at crew show. We eventually found him sound asleep in the base chapel where he had been praying to do just that — to get some merciful sleep.