There are a few answers to that one, and natural solutions. Among the most recommended by holistic doctors are Vitamin D3. We are all deficient in this nutrient, especially if we work in an office, or live in the north, where it's cold, and dreary 6 months out of the year. Sunlight when it hits our skin is converted into D3. An easily assimilated form of D3 is in a liquid form http://www.amazon.com/Liquid-Vitamin-drop-Servings... 200 IU per pound of body weight, under the tongue will bypass body organs, and go straight into the blood stream. A vital nutrient when combating an ailment, including fatigue.
Up the omega 3 fats in the diet, along with B complex will give energy. The b vitamins are necessary to convert the omegas into energy.
Being dehydrated can also be a key factor in fatigue. 1/2 ounce per pound of body weight. Because our muscles require water, dehydration is a contributing factor to weakness as well.
A medical condition referred to as SAD (seasonal depressive disorder) is an ailment whereby the body has become "depressed" because of a lack of sunlight. A light lamp has been found to be very helpful in this condition. We all require sunlight to have "normal" body rhythms. Asleep, and awake cycles, as well as to maintain normal levels of melatonin.
My article http://hubpages.com/health/Saint-Johns-Wort-For-Ma... explains the different approaches todepression, anxiety, SAD, among the other disorders with natural solutions.
Fatigue can also be caused by viral, as well as fungal infections. can also be caused by low thyroid function. Doesn't half to be at a "clinical" low to cause a problem, and fatigue. As Dr Teitalbaum explains fibromyalgia is like "the drop dead flu" His approach is to treat the underlying cause, treat the numerous symptoms, and replenish nutrients. An awesome Dr. I explain his approach in my article. http://hubpages.com/health/Natural-Supplements-for...
It is all trial and error, what works for one may not for another. Keeping a tired diary may reveal the cause.
Start with filtered water, add D3, and B complex. See how much better you feel. Getting outside, even 20 minutes a day, will help. Especially on bright days.