Is it safe to install games while PS4 in rest mode overnight ?

 It's safe but I don't recommend it. It may will causing overheating on your PS4 console, if you repeating that way when installing another games.

It's more better you installing the game for half to one hours, after it PAUSE it then turn off the console after it you go to sleep.

After you woke up you can turn on again your PS4 then RESUME again you had paused the installation before. Repeat all of these steps until all installation is finished.

This way is recommended if the game installation have huge memories but you need be patient.

I did it with my PS4 as well. I hope it's helpful 🙏🏻