Coughing once the baby sleeps is considered to be caused by allergies, post-nasal drip syndrome, gastroesophageal reflux disease and other factors. 1. Allergy: If there are allergens in the baby’s sleeping environment, the baby can inhale them and trigger an allergic cough, leading to a prolonged paroxysmal, irritating dry cough. 2. Postnasal drip syndrome: when the baby suffers from chronic rhinitis, sinusitis and other nasal diseases, the inflammation can stimulate the nasal mucosa, producing a large amount of secretion, when the baby sleeps, the secretion will flow backward to the nasopharynx, thus stimulating the throat, choking cough. 3. Gastroesophageal reflux disease: baby lower esophageal sphincter dysplasia or abnormal function, can trigger gastroesophageal reflux disease, resulting in decreased pressure in the esophagus, baby stomach contents are easy to reflux to the lower end of the esophagus or even to the pharynx, thus stimulating the throat cough. There are many other reasons why babies cough when they go to sleep, so it is recommended that you seek prompt medical attention to identify the cause of the cough and receive treatment as soon as possible.