Disclaimer: this is meant as a joke!
I would get the angry cham out of the cage and chase my brother around the house with him, and let it bite him, to teach my brother a lesson!!
In reality, though, maybe you could cover the cham's cage? Or even get a very cheap security monitor and set it up in your room. Then, you could just check a cell phone app periodically to make sure the lights stay off?
How old is your brother? What's his reason for turning on the light? Just to bug you? Cuz if that's the case, i would get your parents involved. You can't have him stressing out your cham and interrupting it's sleep cycle, just because he thinks it's fun.
I grew up with an older brother. He was a jerk, a lot, just because he could be! Good luck!