The common causes of weakness and lack of energy are physiological reasons, such as over-fatigue, and pathological reasons, such as hypokalemia, anemia, upper respiratory tract infections and so on. 1. Physiological reasons: fatigue may be the problem, after long-term strenuous exercise or heavy physical activities, there may be a soreness, weakness and lack of energy, which can be relieved after resting. 2. Pathological causes: such as hypokalemia, due to improper diet and diarrhea, resulting in excessive loss of potassium and hypokalemia, body fatigue and weakness can occur. Malnutrition in the case of anemia can also lead to body weakness. It is also seen in upper respiratory tract infections, which can lead to muscle aches and weakness. Most of the body weakness can be relieved after resting, such as prolonged weakness can not be relieved, we should go to the hospital in time to avoid delaying the condition.