Medically Reviewed By: There are many different types of headaches, but some of the most frequent and misunderstood ones are tension headaches. In fact, around 90 percent of all headaches can be classified as tension headaches.4 Yet the answer to what causes tension headaches is not always simple. These headaches can be the result of many different factors, including jaw clenching, caffeine withdrawal, long-term stress, smoking, poor posture, and eye strain. Therefore, it is important to determine what is causing a tension headache in order to alleviate the pain and symptoms. Here is an overview of the possible root causes...