Temporomandibular joint disorder. Basically it's what happens when you grind your teeth a lot, especially in your sleep - the muscles go into violent and agonising spasms, and it can feel like you've dislocated your jaw. I actually thought that was what had happened, when it first happened to me. It can be quite serious, some people crack their teeth with the pressure.

The treatment involves wearing a splint over your bottom teeth - it's a bit like a gumshield, and initially you have to wear it all the time. There are also exercises you can do to relax the muscles.