There are various causes of lethargy and drowsiness, mainly the following: First, cerebrovascular disease, cerebrovascular disease in the early stage can appear drowsiness, generalized weakness and other symptoms, serious can appear coma. Second, carbon monoxide poisoning, early manifestation of drowsiness, weakness of the limbs, poor mental and slow reaction. Third, alcohol poisoning, a person who drinks too much alcohol, can appear weakness, drowsiness. Fourth, there are also some drug factors, such as taking sleeping pills to excess, the next day can appear weakness, drowsiness. Fifth, there are also some diseases, such as encephalitis, especially viral encephalitis, autoimmune encephalitis, these diseases can cause the symptoms of weakness and drowsiness.