Okay, Dream magic has literal dream matter (whatever that may be), but that doesn't nullify making it exhausting to try to use your dream matter. I always find it silly when in games somebody casts a bunch of spells and then physically CANNOT for a short amount of time until he has more mana. You don't punch somebody in the face a couple times and realize your punching-ness isn't high enough to deliver the last one. Sure, you may be exhausted, but you don't need to wait before you punch.
And wouldn't it make PERFECT sense that for dream magic, you NEED to sleep to regain power? I mean, it is a dream is it not? Maybe using dream magic doesn't exhaust you as much, but you still have to sleep at the end of a battle to recharge your dream batteries so you can use your dream flashlight to scare some oogy-boogymen.
As regards to the martial arts example, okay. I mean yeah, I need to learn martial arts before I can use my arms and legs like that. Good point. Makes sense. Still though, I can punch somebody in the face without any training. If I had never lived with people and then got in a fight, I probably would instinctively know how to scratch them. If I have magic powers inherintly, would I be able to make lights or pick up stones with my mind? That wouldn't make sense, because then why would we ever need torches or whatever? When you start, you have minimal magic knowledge. Doing something as mundane as the most mundane spell would be impossible, yet punching and kicking isn't.
Also, why is mana "life energy?" What is life and why would it have an energy you can harness? DF really doesn't like life points or assigning numbers to animals, so why would it be like that for mana? If I captured a bunch of hoary marmots and stuffed them in my bag and had a drain mana sort of spell, I'd have what could be a very powerful source of energy. From hoary marmots. A hoary marmot battery.
Okay, let's say that works, but draining such power causes corruption of the user. And? I'm still the player. I can cast as many spells, starve and drain my hoary marmot battery until it's near dead, and be as much a dick as I want, yet I'm still the player. Until I've decided as a player to be evil, what's saying that I am? The people? What, I've cast a couple "throw dilldo" spells at a moose outside their town, but that's not evil. I haven't hurt them. Sure, I'm weird and probably screwed in the head, but not evil.