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!