The muscle, not the joint
The masseter is the strongest muscle in the body for its size. It closes your bite, and it can hold a low-level contraction for hours without you noticing, which is exactly what happens overnight for a lot of people.
By morning you have effectively done a long isometric hold. The ache sits at the angle of the jaw, sometimes at the temple, sometimes as pressure behind the eye, and it fades over the first hour of the day. That pattern is the clue: joint problems tend to worsen with use, muscle problems tend to ease.
Why it shows up at the temple
The temporalis fans across the side of the skull and shares the work of closing the jaw. When the masseter is overloaded, the temporalis takes some of the load, which is why so many people describe a band of tightness above the ear rather than pain in the jaw itself.
The medial pterygoid, deeper and reachable only from inside the mouth, is the third part of that group. It is the one most often missed, and the reason external massage alone can help for a day and then stop helping.
What actually changes it
Manual work on all three, in sequence, with the intraoral portion included. Sessions are forty-five minutes because the external work has to come first. Going straight to the intraoral muscles on a guarded jaw does not release anything.
Six weekly sessions is the usual series, then monthly. If a dentist has made you a night guard, keep wearing it: it protects the teeth. It does not release the muscle.