I agree with binsay, and also let her have the lights on its okay, there is some anxiety make sure nothing new has come up to concern her have you chatted to her about her fears, a lot oif children go through this phase, maybe if you done make an Issue of it it may help. Just accept the phase and relax.
Ok, listen, phobias of the dark can have to do with more than just fearing the dark itself, it can have to do with stress which is hard for some parents to understand, but children do get stressed! Even in the summer. Okay, so 6 isn't really very old and lots of 6 year olds are scared of the dark. I was too and a lamp didn't help me, I had to leave all the lights on. My sleep was fine, and I got all A's in school so leaving the lights on isn't really bad except for the whole enviroment thing. Try getting her an mp3 player or something that plays the music she likes. If she's not focused on the dark it probably won't scare her as bad and she can fall asleep to her music. I think young kids like disney music, so yeah, try that.