My 9 month old baby keeps coughing at night for the past few days and seems to have no cough during the day why is it only when he's in deep sleep?

Post nasal drainage: This is common to kids sensitive to irritants in their environment.Dust, smoke, fragrance, odors, fumes etc all irritate the nasal passages and set up a pattern of drainage. A gentle wash out with saline nasal wash can sometimes help reduce the irritant load.

Runny nose?: His coughing could be due to post nasal drainage (snotty nose) that irritates the back of his throat when he lies flat. Try having him sleep on an incline (always on his back, of course). Humidifiers can help loosen it up as well.

Baby's night cough: Coughing with a cold is often worse when lying down and during sleep because the muscles and tissues of the upper airway become relaxed, mucus drips down our nose and throat, and we don't clear our airway of secretions as well during sleep. However infants with worsening cough or difficulty breathing must be evaluated for more serious conditions, such as pertussis, bronchiolitis, or pneumonia.