how to fix the lighting?!?

[megad3th]

I think there's a problem with the lighting in my game, you don't need flash lights because you can see everything, but sometimes it'll suddenly get bright then becomes much darker, which I think is what the game should look like. How do I keep it that way?
On the left of the pic is what my game looks like and sometimes it becomes like in the right pic for a few seconds, how do I make it stay like in the right pic???