Having a headache can ruin your entire day. And there are various causes for getting headaches.
One related area is the TMJ, or temporomandibular joint. Often times those who clinic their jaw or have tightness in their jaw expereince headaches. There headaches are part of a referral pain pattern coming from tender points in the muscles of mastication.
Clinching of the jaw is also directly correlated to tightness in the neck.
If you clinch your jaw, you clinch your neck. And, yes tender points in the neck can also cause headache like symptoms from referral patterns.