The polysomnographic device in use for this study (System Plus, Micromed, Mogliano Veneto, Italy) measures the oral/nasal airflow by a flow transducer placed below the nostrils, whereas abdominal movements are detected by a band placed around the abdomen which is connected to a respiratory inductance plethysmography. Each oral/nasal flow cessation ≥5 seconds will be considered an apneic episode; apneas are then classified as central (CA: absence of both oral/nasal flow and movements of the abdominal wall), obstructive (OA: abdominal movements with an inadequate oral/nasal flow) and mixed (MA: components of both CA and OA). Apneas included in periodic breathing patters will be ruled out from the analysis. Systemic oxygen saturation is recorded by means of a pulse oximeter sensor, placed on the infant's foot. Cardiorespiratory events will be characterized as hypoxic episodes, defined as the occurrence of SatO2 <80%, and/or bradycardia, defined as pulse rate <100 bpm. The polysomnographic recording is automatically acquired in a computer connected to the polysomnographic device. The number of total apneas, CA, OA, MA and cardiorespiratory events will be traced down and visually analyzed by the sub-investigators.