Some screens are just rubbish, or faulty.
Its actually odd, I just got a 3d/120hz screen, when I setup and watch something 3d if I have my other 60hz normal screen turned on at full brightness I can't see any flicker in the glasses from the other screen, but if I turn the brightness down on the 60hz screen there is visibly more flicker. I don't know if that's relevant, too low brightness can be a problem as well maybe, both because screens aren't tuned for low brightness and while you think high brightness is the problem maybe you've turned it too low and you're straining due to low brightness now.
I'd try an optician though, nothing worse than screens that cause headaches, I suffer from migraines which can easily be caused by watching a monitor too long and definately from a really crappy monitor or something like a low refresh rate CRT, which isn't such a problem these days as you rarely bump into one anywhere you might use a computer now.
As for cpu power/heat, there isn't anything out there application wise that should lower brightness and use a huge amount of cpu power to do so, so whatever that is, get rid of it.
As others have said, I've not seen a laptop in as long as I can remember without brightness options somewhere, usually function key + whichever keys also have a lamp type symbol on them(often cursor keys).
