Sleeping in Hair masks?

I was at the local Walmart shopping for hair masks, i always read the directions in the store because I want to make sure its safe for my roots and one of the hair masks said either to use it the normal 20 minutes in the shower or overnight for 8 hours. I know you shouldn't sleep with oil in your hair and on your scalp that long, but has anyone done it with a hair mask and what did you think of it? Was it any different than the 20 minutes in the shower or did it damage your hair ?