Sudden drowsiness in 3-month-old babies is common in the following cases: 1. diabetes, because diabetes is present but not detected in time, when ketoacidosis occurs, drowsiness is likely to occur, so you need to go to the hospital in time for blood sugar and other related checks. 2. hypoglycemia, the baby is not given sufficient breast milk, thus lowering blood sugar. 3. neurological disorders, when there is encephalitis or intracranial hemorrhage and other diseases are also prone to sudden drowsiness. 4, poisoning, if inadvertently ingesting drugs or other toxins, drowsiness is likely to occur, in short, regardless of the cause of drowsiness are more serious, need to go to the hospital in a timely manner.