The lethargy is feeling of lack of energy or tired or drowsy at times.Depression or stress is the most common cause of it.Other causes includes poor or mal nutrition,high or low BP,thyroid or other adrenal gland disorders, diabetes,lack of sleep,fibromyalgia,anemia etc.Lack of concentration,sleep disturbances,mutiple joint pain,body ache,change in appetite and bowel patterns are the symptoms of lethargy.