The exact cause of tension headaches is unknown. Researchers have developed a number of theories.

For many years, researchers believed tension headaches were caused by the contraction of neck and scalp muscles. Recent research, however, has cast doubt over the accuracy of this theory. A test known as electromyography has been used to record electrical currents produced by muscle activity in people diagnosed with tension headache. The test has been unable to identify increased muscle tension in these people. This indicates that muscle tension may not be the main cause of tension headaches. As a result of these findings, the International Headache Society has begun to refer to tension headaches as “tension–type headaches.”  

A new theory that has emerged suggests that tension headaches are the result of changes in brain chemicals responsible for communication between nerves (e.g., serotonin, endorphins). Although scientists do not understand why these chemical changes take place, they believe the process stimulates pain pathways to the brain and hinders the brain’s ability to suppress pain.

Nitric oxide (a substance involved in the transmission of nerve impulses) is another chemical linked to tension headaches. Researchers believe overproduction of nitric oxide is related to chronic tension headaches. In addition, substances that prevent the production of nitric oxide have been linked to a reduction in the muscle tightness associated with tension headaches.

The chemical changes related to tension headaches are similar to those associated with migraines. As a result, some experts believe the two types of headaches are related. Although the connection is unclear, some researchers believe migraines result from the regular occurrence of tension headaches. As the tension headache–related pain increases in intensity, the characteristics associated with a migraine appear. Another theory is that mild migraines are a form of tension headache.

A number of factors may trigger a tension headache, including:

• Body positioning. Poor posture and ergonomics, working in awkward positions, holding one position for an extended period of time, keeping the head or neck in an unnatural position.
  
  
• Lack of physical activity.
  
  
• Fatigue.
  
  
• Lack of sleep or changes in sleep patterns.
  
  
• Sleeping in a cold room.
  
  
• Skipping meals.
  
  
• Eye strain or close work under poor lighting conditions.
  
  
• Psychological conditions such as stress, depression or anxiety.
  
  
• Hormonal changes such as those caused by menstruation, pregnancy, menopause or hormone use.
  
  
• Medications. These include antidepressants, antihypertensives (drugs for high blood pressure) or frequent use of analgesics (creating rebound withdrawal headaches). However, research has shown that antihypertensives can also prevent headaches in some cases.
  
  
• Other illnesses. Arthritis, sinus infection, nasal congestion, colds or influenza.
  
  
• Physical abnormalities. Neck muscle, bone or intervertebral disc abnormalities or misalignment of the teeth or jaw.
  
  
• Sensory stimulus such as light or noise.
  
  
• Stimulants such as alcohol, nicotine or caffeine.
  
  
• Overexertion.

In addition, a number of factors appear to make a person more likely to develop tension headaches. These risk factors include:

• Age. Although tension headaches can occur at any age, they are most common in patients between the ages of 30 and 39.
  
  
• Sex. Tension headaches are more common in women than men.
  
  
• Family history. Tension headaches are more likely to occur in people with tension headache sufferers in their immediate family (parents, siblings, children). This may be the result of an inherited susceptibility.
  
  
• History of migraines. Most migraine sufferers also get tension-type headaches.