During the 24-h pH-MII monitoring each infant will receive eight meals, four of which will be followed by NNS treatment with a pacifier. The sequence of NNS application will be consecutively alternated in study participants. For each evaluation, a minimum of 2 hours of NNS post-prandial treatment is required. The healthcare assistants (i.e. medical staff and nurses) will inspect the correct application of NNS, recording the effective time of pacifier usage and regularly checking the pacifier position in relation to the pH-MII probe and polysomnographic flow sensor, to avoid possible interferences. Due to their immaturity, the enrolled patients might show an ineffective sucking, potentially losing the pacifier. In this case, the healthcare assistants will offer the pacifier to the babies up to a maximum of four times for each postprandial period; measures to fix the pacifier will be avoided.