You can't forget that DFRP uses mental stress for fatigue (well at least whilst casting) a sleep spell could easily be seen as a fatigue spell.
That is, because it is a mental exercise. If you were to run a few miles, I am pretty sure that would be physical stress.
I know that phenomenon when in a high studying phase. I would learn all day and be tired as hell after that. But I didn't do anything physical, and I would be able to do a whole lot more, as long as it didn't involve thinking. And I know this the other way, too, though being physically exhausted will have more of an impact on your mental capacities than the other way around.
So if you are trying to tire someone, you could use this to your advantage (or it could be used to your disadvantage). A huge guy, made mostly of muscle might not be able to withstand much mental stress and could be worn down like that. On the other hand, he is probably not using much of his brain when he is punching you in the face, so he might be "immun" to that kind of sleep spell (which might be reflected in an aspect like "all muscle, no brain" or even a stunt).