A portable pulseoximeter (WristOX2 3150, Nonin) is used for ambulatory NPO. NPO is carried out the day immediately before or after the PSG at patient's home. Patients are assigned to carry out the NPO study before or after in-hospital PSG randomly. In addition, oximetry is also performed simultaneously to the PSG by means of the portable pulseoximeter. Therefore, every patient has 3 oximetric recordings: (i) SpO2 from unattended portable monitoring at home, (ii) SpO2 from attended in-hospital portable monitoring and (iii) SpO2 from attended in-hospital standard PSG.