Why is my jaw locking when I first wake up?

Locking. If you clench or grind, you may have muscles that are cramped. If you have a dysfunctional joint or joints, you may have displacement of the disk within the joint. Please see your Dentist who may consider placing you in an Orthotic to wear when you sleep, or who may want you to see an Oral Surgeon, Orthodontist, or a Dentist who concentrates in treatment for relief of int pain.
Morning stiffness. Clenching, bruxing or grinding teeth during sleep can exhaust jaw muscles and lead to pain, stiffness and morning jaw locking. See Orofacial Pain specialist for TMJ evaluation and treatment.
Jaw locking in a.m. If your jaw is locking closed, it generally means that the disc in the jaw joint may be displaced. If your jaw is locking open, that means that the jaw is itself becoming dislocated, which is much less common. For a closed lock that fails to respond to simple measures, it has been shown through many studies that the most effective treatment is TMJ arthrocentesis or arthroscopy, to unlock the jaw.
Disk. Jaw Joint (TMJ) disk out of position. Indicates advanced problem. See TMJ expert. If locking progresses, you may need a TMJ arthrocentesis or arthroscopic surgery.