Doctors don't know what causes fibromyalgia. Current thinking centres around a theory called "central sensitization." This theory states that people with fibromyalgia have a lower threshold for pain because of increased sensitivity in the brain to pain signals. Researchers believe repeated nerve stimulation causes the brains of people with fibromyalgia to change. This change involves an abnormal increase in levels of certain chemicals in the brain (neurotransmitters) that signal pain.

In addition, the brain's pain receptors (neurons), which receive signals from the neurotransmitters, seem to develop a sort of memory of the pain and become more sensitive, meaning they can overreact to pain signals. In this way, pressure on a spot on the body that wouldn't hurt someone without fibromyalgia can be very painful to someone who has the condition. But what initiates this process of central sensitization isn't known.

It's likely that a number of factors contribute to the development of fibromyalgia. Other theories as to the cause include:

  • Sleep disturbances. Some researchers theorize that disturbed sleep patterns may be a cause rather than just a symptom of fibromyalgia.

  • Injury. An injury or trauma, particularly in the upper spinal region, may trigger the development of fibromyalgia in some people. An injury may affect your central nervous system, which may trigger fibromyalgia.

  • Infection. Some researchers believe that a viral or bacterial infection may trigger fibromyalgia.

  • Abnormalities of the autonomic (sympathetic) nervous system. Part of your autonomic nervous system — the sympathetic, or involuntary, system — controls bodily functions that you don't consciously control, such as heart rate, blood vessel contraction, sweating, salivary flow and intestinal movements. It’s thought that sympathetic nervous system dysfunction occurs in people with fibromyalgia, particularly at night, which leads to fatigue, stiffness, dizziness and other signs and symptoms associated with the condition.

  • Changes in muscle metabolism. For example, deconditioning and decreased blood flow to muscles may contribute to decreased strength and fatigue. Differences in metabolism and abnormalities in the hormonal substance that influences the activity of nerves may play a role.
Psychological stress and hormonal changes also have been considered as possible causes of fibromyalgia.