This simple and beginner friendly guide shows the basic principle of an night detector for your indoor lightning.
Just to make clear; this is just a way to show how it works. As you can see the setup is simple and not recommended. Check out Automatic Battery Backup for a better power setup.
Whole idea is to use Large Solar Panel combined with Counter and a Blocker to trigger power blocking on daylight. When the sun goes down the power decreases and when power is below 15 (Counter target power in this case), the Blocker sends power through again.