You may want to check medical dictionaries. They do classify fatigue as connected to various illnesses mostly.
But you can also find a lot on driving and flying related issues.
Leaving very early and having to do a 12 hour day will make you tired, very tired. In my case, it will make me fatigued, too. It is very much against my natural rythm, and during a 5 day schedule of "early shift", my sleeping pattern changes to sleeping 3 hours in the afternoon and another 3 from about 12.30am to 4am or whenever I have to get up.
However, I personally am not that affected by a late rythm, say start duty at 1pm and work till midnight. Or the ones where you fly through the night, just like the schedule on which the Mangalore crash happened. Lots of my collegues hate that kind of duty, though.
I am simply very tired in the morning, but far from fatigued.
The simple trick would be to let pilots put in a preference with the crew scheduling department (they do at my company) and plan accordingly (they sure don't at my company). Finacial interest has made them decrease the number of employees there, and now they just have the time to get the rosters out, there is no capacity to look at the indivdual.
So I do believe my schedule could be much more suited to my sleep, as weired as that my sound to a non aviation person. Also a lot of social problems (single moms working as flight attendents) could be avoided, but that would mean more cost/less profit by hiring another planner.
But I believe in realizing your weakness (getting up early in my case), and acting accordingly let's one reduce the impact on flight safety, and a pilot caught snoring on the CVR (as spoken of at the Mangalore thread) is not necessarily a bad one...
Nic