What causes a morning tension headache?

Unknown! Tension headaches are characterized by dull, non-pulsatile discomfort on both sides of the temples and forehead. Typically last for 30-60 min or for days. Affect at least 40% of adults in any given year. Occur most commonly in the early morning and late afternoon. The morning variety is often triggered by lack of sleep, awkward sleeping positions, hangovers and caffeine withdrawal. Cause unknown!
Headache. Your headaches are coming from tension, dehydration, lack of sleep/poor sleep, teeth grinding, skipping meals, stress, sleep apnea/snoring, or chronic sinus problems.

Related Questions

Morning eyelids swelling, a little headache. Doc said it was tension headache, but what causes the swelling? I wake up with it & during day gets better

Maybe nothing. We all have a little facial swelling while sleeping that improves once we get up and move around, due to the recumbent position of sleeping. You might try sleeping on 2-3 pillows to elevate your head; this should make the swelling less of a problem.
Sleep on face? Often orbital fat which has prolapsed into the eyelid will collect fluid due to gravity during sleep.