I am Dr Shamoon. The pattern you describe in your 1 year 7 month old daughter - a cough only at night, with a completely symptom-free, active and playful child through the day, no fever and no breathing difficulty - is very typical and usually points to one of three things. First and commonest, post-nasal drip: mucus from the nose and adenoids trickles down the throat when she lies flat and triggers coughing bouts. Second, cough-variant asthma or allergic airway sensitivity, which classically worsens after midnight and in the early hours, and is more likely if there is asthma, allergy or eczema in the family or if she has had wheezing episodes before. Third, silent reflux, which is common at this age, especially if her last feed or milk is close to bedtime, and gives night cough with occasional gagging. Dust, mosquito coils, incense, room fresheners, soft toys, a fan blowing directly on her, or a cold air-conditioned room can all aggravate all three. Since she has no medicines, no diagnosed condition and no known allergy, and gains no daytime symptoms, this is very unlikely to be a chest infection or tuberculosis. What decides the treatment is the character of the cough (dry versus wet), whether you hear any whistling sound at night, and whether her nose is blocked or snoring is present - a physical examination of the chest, nose and throat is what will settle this rather than trying more syrups.
Next Steps
1) Elevate the head end of her mattress by about 20-30 degrees (place a folded blanket under the mattress, not a pillow under her head) - this alone helps both post-nasal drip and reflux. 2) Give her last milk or meal at least 1.5-2 hours before she sleeps, and no bottle feeding lying down. 3) Saline nasal drops 2 drops in each nostril at bedtime, and steam inhalation in the bathroom for 5-10 minutes before sleep (never direct steam to the face). 4) Half a teaspoon of honey at bedtime is safe and effective at her age for night cough. 5) Reduce night triggers: no mosquito coil, incense or spray in her room, remove soft toys and carpets, wash bedding in hot water weekly, avoid the fan or AC vent blowing directly on her, keep the room at a comfortable temperature and not too dry. 6) Keep her well hydrated during the day. 7) Note for 3-4 nights: is the cough dry or wet, is there any whistling/wheezing, snoring or mouth breathing, does she vomit with coughing - and take these notes to the doctor. 8) Get her examined by a paediatrician if the cough continues beyond 2 weeks, or earlier if it is worsening; if asthma-type cough is suspected, a short inhaler trial with a spacer and mask is the standard next step. Avoid buying over-the-counter cough syrups - they are not recommended under 2 years. You are welcome to consult me on Practo for a detailed review, or in person at Synergy Multispeciality Hospital, Miraj.
Health Tips
Word of caution: take her to a doctor immediately if she has fast or laboured breathing, chest indrawing, a whistling or barking cough, bluish lips, high fever, refusal to feed, unusual drowsiness, or if the cough started suddenly while she was playing with a small object (possible foreign body). Do not give her adult or over-the-counter cough or cold syrups, and never give any antibiotic without a doctor's examination.