Vitamin D deficiency is a condition where you aren’t getting sufficient vitamin D to stay healthy. Vitamin D is naturally produced from cholesterol in your body on exposure to sunlight. It can also be gotten through food and vitamin D supplements.
Every cell in your body has a receptor for vitamin D. This vitamin is essential for the development of strong bones. It helps your body absorb calcium, which is an important mineral in the bone. Vitamin D also plays an important role in proper muscle, nerve, and immune system function.
A deficiency in vitamin D may be caused by a lack of sunlight exposure or poor dietary choices. It may also be because your body fails to properly absorb vitamin D or because your kidney or liver cannot convert vitamin D to its active form in your body. Certain medications can also interfere with your ability to use vitamin D. 
Vitamin D deficiency is common, affecting about one billion of the world’s population.  A 2011 study found that it affects around 41.6 percent of adults in the United States.  The numbers are higher in Hispanics and Black Americans at 69.2 and 82.1 percent respectively.
Many people do not know they are deficient in vitamin D because the symptoms are usually nonspecific. Read on to find out some of them.
Fatigue is a symptom of many health conditions. It is nonspecific, so very few people will suspect vitamin D deficiency to be the cause. But studies have shown that a deficiency of vitamin D in the blood can cause fatigue that may interfere with one’s normal life. 
Observational studies, such as one carried out with female nurses who experienced intense fatigue, have established connections between vitamin D deficiency and excessive tiredness. 89 percent of the nurses in the study were found to be vitamin D deficient.  Another one with young women revealed that those whose vitamin D blood levels fell below 30 ng/ml, were more likely to complain of fatigue than those who had more vitamin D in their blood. 
Taking vitamin D supplements can help raise the levels of vitamin D in the blood and reduce fatigue. A woman who had only 5.9 ng/ml of vitamin D in her blood and complained of headaches and terrible daytime fatigue was helped by taking supplements that raised her vitamin D levels to 39 ng/ml.