I'm actually quite leery of foriegn studies... there may be significant differences in rest rules, and route structure. North-South vs. East-West obviously matters a lot...

Also how do you segregate long haul from short haul? In the US you could use NB only airlines for shorthaul, and some cargo for longhaul but at the big three most pilots will mix and match fleets over their career.

If your short haul pilots all work for lower-tier airlines, maybe they have less rest due to contracts, more stress, less money, etc.