The causes of sudden drowsiness in 4-month-old babies are as follows: 1, to see if it is due to environmental factors, such as high ambient temperature, such as the hot summer, the baby will suddenly appear drowsy. 2, is not due to disease, the child has recently eaten poorly, there is no diarrhea and other dehydration phenomenon occurred. If the baby is dehydrated due to insufficient energy intake, the baby will appear drowsy. This situation should be actively given to correct the cause of diarrhea, correct the symptoms of diarrhea, if necessary, give rehydration energy support treatment, drowsiness can be improved. 3, due to the baby vitamin D as well as calcium supplementation is insufficient, vitamin D deficiency leads to impaired calcium absorption. The two together lead to pediatric plant nerve dysfunction, while the lack of calcium children sleep insubstantial, easy to have awakening and other phenomena. This child is not well rested, will appear drowsy phenomenon after a long time a state, need to take the child to the hospital for trace elements and vitamin D testing. 4. Observe whether the baby has the symptoms of cold recently, if there is a cold or disease state occurs, the phenomenon of encephalitis may be formed. According to the above specific performance of the child, a comprehensive analysis is needed.