Leaving hair conditioner in your hair overnight is perfectly fine and no harm will be done if you do leave any conditioner in your.
Just be sure to rinse the hair conditioner out of your hair first thing in the morning and nothing bad will happen to your hair.
You can buy a type of hair conditioner that is made for leaving in your hair overnight and it's called leave in conditioner which is even better for your hair than normal hair conditioner.
Some people say they damage their hair when leaving conditioner in too long or overnight but as long as you use the leave in conditioner or don't put too much conditioner in your hair overnight then you should be just fine.