Can someone confirm at witch level of stamina those status are triggered (0-25-50-75%)? That information seems to be missing in the Phoenixpedia… and it would save time to everybody not having to make experimentation to figure it out.
I may be wrong but I think like that:
tired <= 9 stamina
exhausted = 0 stamina
Thanks for the info. Have you gone to 10 with no lose of action point?
I went even lower, but I’m not sure what is the limit. Probably had 10 at some point, but I don’t remember if that mission had change in AP or not.
Below 20% you suffer an AP penalty according to the tutorial. That would be 8 or below tired, and probably 0 exhausted.
This question helps me greatly because I’d misread and thought it said 20 not 20%. I’ve been resting way too often.
I had the -1AP penalty once and it was very low, 8 makes sense it should be that.
Sometimes a soldier get more fatigue I failed figure why, very rare, it was perhaps a bug.
Never tough it was that low 
Made some tests and at 12 there is no penalty. The next mission got a message box to warn that there was tired or exhausted soldier(s) in the squad. Many of them were at 7. In the mission, soldiers with 7 had a -1 AP penalty. So 8 could be the magic number.
Base stamina is 40, 20% of 40 is 8, so it’s definitely 7 or 8 that the -1AP penalty happens.