Going to bed without giving your hair time to dry is something many women do, but this habit can actually really damage your hair. How?
Sleeping with wet hair can cause headaches and weaken your immune system. Additionally, your pillow will get wet, which makes it a perfect setting for bacteria development. This won’t happen over one night so there’s no need to panic in the morning, but make sure you don’t leave your pillows wet all day. Leave them in the direct sun to dry before the next use.
Your hair is the most prone to damage when it’s wet — you already know you shouldn’t brush it and pull it then. As you turn around in your sleep there’s a big chance you will damage the hair — but this also doesn’t happen over the course of one night. A month or two of regularly going to bed with wet hair will probably cause a change in its quality.
In conclusion, sleeping with wet hair once in a while won’t do anything drastic, just make sure it doesn’t become a habit.