“Is it bad to leave toothpaste on your teeth overnight?”

Not recommended

Never let toothpaste in your gums and teeth overnight, because the detergents and the fluoride can harm your soft and hard tissue. By the way, that's why biological dentists recommmand fluoride free toothpaste.

Great questions because it has two big pieces: 1. Depending on the abrasion index of the toothpaste which is nowhere to be found on the tube (which it really should be), you should never leave an abrasive paste with index over 100 in your mouth. It can be detrimental especially if you are a grinder. It becomes like sandpaper destroying your teeth. 2. If the paste contains tarter or chemicals like whitening agent, that can be irritating to your soft tissues, which is also bad. A paste with CLO2 or another type of antibacterial is okay. It may actually be good. As well, a fluoride-containing paste has some beneficial effects as does xylitol-containing.

Don't do that. The toothpaste should be brushed off immediately and then go to bed.

If you have flossed and brushed your teeth you have plaque, bacteria food particles, in that tooth paste, you might feel better rinsing your mouth.
Every once in a while, they study what's in tooth pastes. There are not suppose to be anything that can cause cavities in your tooth paste. It might be a good idea to rinse it off.