just a warning:
be careful of using dental nightguards. if you have any TMJ symptoms at all, a nightguard can exacerbate them. and, trust me on this one, TMJ pain is worse than any annoyance or inconvenience cause by regular old clenching & grinding.
i would strongly advise seeing a TMJ specialist if you have any pain in the joints in your head, which can radiate to include the entire muscular structure of the cranium, neck & shoulders.