Interesting fact:

Do you know why we call bad dreams ‘nightmares’? The term comes from the Old English and has been in use since 1300. ‘Mares’ were mythological demons or goblins who tormented people with scary dreams. They were believed to ride horses all night and return home in the morning exhausted and perspiring. Mares would also tangle the hair of humans during their sleep. So now you know the reason for that bed head!