2018 Prime Question

I have to check it to be sure, but I think you are OK in 2018 PP to keep it on READY mode overnight, provided you have plenty of gas. If the traction battery has some charge, then it will stay on READY without the engine starting until you run out of charge. The time will depend on how much charge is being used by A/C.

Toyota did change the safety feature on PP over the years, and on my current 2021 PP there is a safety feature to shut off the system after 1 hour in P. You can override this automatic "shut off" by either depressing the brake pedal or shifting in other than P to reset the timer to zero (will have to repeat every 1 hour). Or if you are sleeping in a car, then more conveniently, by just locking the door from the inside or using the mechanical key to lock the door from the outside, if you are not inside of the car. This will disable the auto shut off feature.

The following is the description of this feature on my 2021 PP. But as I said, I think on the 2018 model, this feature did not exist, so it should stay READY indefinitely.

EDIT: I checked the 2018 PP Owner's Manual here. 2018 Toyota Prius Prime Manuals & Warranties | Toyota Owners
In there, I could not find any section that contains an Automatic hybrid system shut off or alike feature on the 2018 PP. I also checked the 2020 PP manual and did not find any section about this feature. I am now almost certain that this feature started in the 2021 model year and most likely be found in the 2022 models too. But it can be easily disabled as stated in the manual below.