What does where the pain lies tell you about the nature of a headache?

Most headaches around the eyes, cheeks, temples, and top, back or base of the skull involve the muscles, connective tissue (myofascia), nerves or structures around the joints of the jaw, ear, and/or sinuses. These headaches can be primarily caused by a direct strain or sprain, or secondarily caused by drug side effects or other conditions that precipitate inflammation in those areas. The brain tissue itself does not have nerve endings that perceive pain, but the tissue surrounding it certainly does!

There are many muscles in the neck and head. I’ll primarily address the myofascial pain pattern of each one, since their are too many related conditions and drug effects to list in a single answer. In any case, a thorough examination by a physician to determine the cause of the pain would still be called for.Areas of pain in muscles (from Travell and Simons’ Myofascial Pain and Dysfunction, volume I) are grouped into the following areas:

Varieties of headache (from Travell and Simons' "Myofascial Pain and Dysfunction," volume I)

(Ch.5, pg.167)

The muscles that create the referred pain in these areas are grouped here, in the same text:

Travell and Simons' "Myofascial Pain and Dysfunction," volume I

If you are curious about the specific pain pattern of a particular muscle, you can visit this site (or other similar accredited sites) to find out: National Association of Myofascial Trigger Point Therapists.

For the temporalis muscle, a sample pattern might look like this:

Temporalis muscle

Muscles of the neck can refer pain locally or up into areas of the head, including the face and jaw. Most facial muscles (including muscles of the eye, ear and jaw) tend to refer pain very locally; they very rarely refer down to the neck.

Pain in the face and jaw, especially, tend to indicate problems with the sinuses, eyes, muscles of mastication (chewing), dental problems (such as malocclusion or infection), or some other local infection (such as a cold or flu). Pain can be triggered by any recurrent infection or source of inflammation, even cancer. This essentially means that therapy for the muscles may provide temporary relief, but since the primary trigger has not been resolved, it will keep recurring.

If the problem is primarily myofascial, correction of any source of mechanical strain (including postural or ergonomic strain) must be removed or at least reduced for the therapy to be effective and lasting.  

The first 3 books on this list are my favorites on non-migraine headaches for the layperson: Amazon.com: trigger points and headache: Books

BUT, of course, if a headache is especially severe, has lasted for longer than three days, happens repeatedly, is interfering with your functioning significantly, is affecting your eyesight, hearing, balance, cognitive function, or is doing anything other than just making your head hurt, please go see a doctor right away!

Are dental guards and occlusal splints really effective to treat Bruxism (teeth grinding)?

As long as the guards / splints are well designed and worn consistently, they can be effective for those people who grind so badly that they damage their teeth. Ill-fitting splints, however, can create more problems than they solve (besides which, they are rarely worn because they are uncomfortable).

In my practice, the most common problems I’ve seen with splints is that they are too big. This throws off the placement of the temporomandibular joint (TMJ) which then makes the muscles of the jaw (most notably the masseter and pterygoid muscles) painful and tight. Very often, the tension in these muscles feeds into the grinding through muscle tension and heightened activation.

In the below diagrams, the red areas indicate the primary patterns of pain associated with bruxism. In the first diagram, the muscle drawn at the back of the jaw is the masseter, which is the main chewing muscle. You can easily feel this muscle when clenching your jaw:

Masseter muscle

Bruxism also affects the medial and lateral pterygoid muscles:

The medial and lateral pterygoid muscles

which are very difficult to reach, but important to treat for TMJ pain and bruxism.

There is a good product out now for self-treatment of the muscles of the jaw, called The MyoFree Solution:

The MyoFree Solution

Very often, if you deactivate trigger points in the jaw and restore normal tone and resting length, the bruxism reduces or is eliminated entirely.

In the meantime, though, let your dentist know that the splint is uncomfortable, and ask them to reexamine the fitting. Good luck!