context: Everyone feels fatigued sometimes, and many people feel fatigued a lot of the time. But, chronic fatigue syndrome isn't just being tired. It's a new state of fatigue that has lasted for at least 6 months, and that can be so severe that it gets in the way of your normal daily activities, at home and at work. Rest and sleep don't seem to help. If you have it, physical activity can leave you feeling worse, typically the next day, a condition called post-exertional malaise. You tend to awaken almost every morning feeling as if you did not get enough sleep, and often you wake up a lot at night, for no apparent reason. You can have trouble concentrating and multi-tasking. When you stand upright from a sitting or lying position, you can feel lightheaded, and your heart can beat rapidly. After you are on your feet for a while, you can feel terrible, and need to lie down flat. Chronic fatigue syndrome is also known as myalgic encephalomyelitis ( ME/ CFS). It tends to cycle through flareups and remissions, good days and bad days, although on the good days you are not back to normal. There isn't any known cure, but different treatments can help the symptoms. The CDC estimates up to 2.5 million Americans have ME/ CFS. Anyone can get it, including children and teens. It is most common in women in their 40s and 50s. Women are more likely to develop it than men. Most cases are mild or moderate. About 1 in 4 people with the condition have severe symptoms. If you have mild chronic fatigue syndrome, you can probably manage on your own. Moderate symptoms can make it hard for you to move around. For example, you might need to sleep in the afternoons. If your symptoms are severe, the impact on your quality of life and abilities can be as bad as if you had lupus, heart disease, or rheumatoid arthritis. Doctors don't know the cause of chronic fatigue syndrome, but they have identified different underlying abnormalities in people with the condition. Immune system problems: Several different parts of the immune system are different in people with chronic fatigue syndrome, and some research indicates that these abnormalities may cause the symptoms of the illness. However, fortunately, people with chronic fatigue syndrome do not have a defective immune system, in the way that people with HIV/ AIDS do. Energy production: In people with chronic fatigue syndrome, the cells in the body have trouble making enough energy. Brain abnormalities: Abnormalities are seen in pictures of the brain (like MRI or CT scans), in levels of brain hormones, and in the electrical system of the brain (brain waves). These abnormalities can come and go, and are not necessarily permanent. Blood pressure and pulse issues: On standing, people can have a drop in blood pressure and an increase in how fast the heart beats. Sometimes people feel like fainting or actually faint, if the blood pressure drops too low. Genes: Some studies have found abnormalities in the structure of certain genes. Other studies have found abnormalities in how some genes are turned on and off, inside cells. Studies of identical and non-identical twins indicate that some people inherit a genetic susceptibility to getting the illness. Infections or other illness: Chronic fatigue syndrome often, but not always, begins with a sudden infectious-like illness (fever, sore throat, aching muscles, upset stomach). Research has found that several different types of infectious agents can trigger the beginning of the illness, including Epstein- Barr virus (a common cause of mononucleosis), Lyme disease bacteria and Q fever bacteria. Serotonin and cortisol: Several studies indicate that serotonin, a major brain chemical, is important in the symptoms of chronic fatigue syndrome. People with the illness have low levels of cortisol, a hormone that the body releases in response to stress. The National Academy of Medicine has issued guidelines describing the combination of symptoms necessary for doctors to diagnose the condition. Unfortunately, there is not yet a diagnostic test that is sufficiently accurate to be useful. Because extreme fatigue is a symptom of so many conditions, your doctor will want to rule out other conditions first, before considering a diagnosis of chronic fatigue syndrome. You'll need to get a complete checkup and talk with your doctor about all your symptoms.