The connection between anxiety and fatigue is that one often causes the other. If a person is experiencing frequent anxiety or stress, the body is flooded with stress hormones, particularly cortisol; this can lead to fatigue, as well as make it difficult to sleep. Conversely, if the person is already fatigued from not getting enough sleep at night or from working too hard, this can cause anxiety because energy reserves are depleted.
Stress hormones are quite damaging to the body. They are intended to produce a fight-or-flight response when they are released, but when stress is constant or perpetual, the hormones build up in the body and can raise blood pressure and anxiety levels. This constant exertion on the body can quickly lead to fatigue, while at the same time making it difficult to sleep at night because the stress hormones are still in the body. This is the best example of how anxiety can lead to fatigue.
Anxiety and fatigue are two symptoms that are frequently found together, unfortunately. In addition to simply causing each other, they are common symptoms of some other mental illnesses such as depression or bipolar disorder. People who are depressed frequently experience anxiety and fatigue in addition to their other depression symptoms.
Some medical conditions could cause anxiety and fatigue as well. Thyroid conditions, for instance, frequently affect one's energy levels and can cause mood disturbances, as well as appetite disturbances and other symptoms. Other medical conditions such as fibromyalgia cause these symptoms as well, however, so if the symptoms persist, it is best to visit a doctor to rule out any potential illnesses. Anxiety and fatigue are common symptoms, so it is likely that blood tests or other diagnostic evaluations will be required if no other indicative symptoms are present.
If the anxiety and fatigue are occurring without any medical basis, it will be necessary to take steps on one's own to boost energy levels and reduce stress or anxiety. It may be helpful to narrow down the cause of the anxiety, if possible; for instance, work stress is a leading cause of both anxiety and fatigue. Speaking to a therapist can be helpful, and can help people develop coping tools for dealing with stress. In addition, trying to boost energy by eating a healthy diet, getting some exercise, and getting enough sleep at night can reduce fatigue, thereby reducing anxiety.