I personally never turn off my internet modem or WiFi at night or ever and the only time my internet modem gets turned off is if I'm on vacation for a few weeks or more or when the power goes out.
As long as you have a good secure password on your WiFi connection then there's no need to shut off the WiFi signal at night.
So you can shut the WiFi off at night if you're worried about hackers or someone hacking into and using your WiFi at night but I wouldn't worry about it.
It's too much hassle for little gain if any gain.