MitD X: If I told you, you wouldn't believe me

[Grey_Wolf_c]

he's absurdly strong and physical effort doesn't seem to fatigue him.

Incorrect. He is tired after the earthquake, and hungry and possibly tired after the Tower scene.

rweird already covered most of the bases: sure, it feels like a clue. But 3.5 doesn't really have either rules for being tired, nor creatures with powers that can only be used X times per period (that is more a 4E thing). Furthermore, Elan, a human bard, has been known to get tired too, even though he definitely has no reason for it (having spent the day sitting on a cart) so maybe it is just an OotS realism thing. Using your powers just takes a lot out of you and when you are low in motivation like MitD is, well, it just doesn't take much to put him back to bed.

Grey Wolf