They cut the brightness down quite a bit. I know someone that leaves them on all the time, night or day, but I used to take mine off when it got dark (they are Velcro'd on, so they're easy to take off and put back on. If you have fog lights, that will help at night too, if you don't want to keep taking them off when it gets dark. I've since switched to clear covers (cut from plexiglass from the hardware store) and pinstriped them, so I don't have to take them off all the time. I did this because I didn't put one on good enough and it blew off somewhere. I like the way they make your car look and I even had a strange feeling when I put it on that morning, one didn't feel quite right, but I didn't re-do it, and sure enough, that's the one I lost.