Leaving conditioner in the hair overnight is okay if the hair needs a deep moisturising conditioning, but use an organic conditioner. However, there are hair masks available that will give better results in 20 or 30 minutes. They are more expensive but can work well.
Leaving conditioner in the hair won't hurt the hair, but what is in the conditioner may cause a reaction on the scalp so it's best to use organic conditioner. To leave conditioner in the hair overnight, coat every strand of hair and wrap the hair in plastic to keep moisture in and conditioner from getting on bedding. In the morning the hair will feel stiff and should be rinsed with warm water thoroughly. Make sure that none of the ingredients in the conditioner will cause an allergic reaction by doing a test in a small area the night before.