Don't you have any fullbright hck for dayz?
Something like that https://img801.imageshack.us/img801/6743/wwpkjzhdb3a.jpg
try increasing your post processing, it helps alot. but in the last patch they lowered light levels anyway, so its pretty dark at night now, even with adjusted settings
If it just got dark then you can use this setting but if it's midnight i don't think that will help. Just spawn nightvision.
It also depends on the moon.
Arma 2 has proper moon cycles, when it's full, it's bright, new moon means almost total darkness.
Hack in NVGs if it's too dark?
There's a hack for that.