I'd go ask your eye doctor what they think. They can give you a much more educated answer, and handle any follow up questions you may have.
I will say that I don't think the brightness level will have anything to do with the level of eye strain you endure. However, that's just my lay opinion, which has no real medical background to back it up.
i want to buy a ds lite but im worried that my eyesight may worsen by staring at the screen. But if i play the ds on full brightness, will that decrease my chances of worsening my eyesight.Also, i want 2 play for about 3 hours a day. And finally, my eyesight is 700 for both eyes. ( Yes, I know ... my eyesight sucks...)

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic