Treatment of Patients With the Obstructive Sleep Apnea Syndrome at Altitude

Inclusion Criteria: * Obstructive sleep apnea syndrome, successfully on CPAP therapy * Residence at low altitude (\<800m) * Obstructive apnea/hypopnea index \>20/h and a complaint of excessive daytime sleepiness before introduction of CPAP therapy. * \>15 oxygen desaturations/h (\>3% dips) during an ambulatory nocturnal pulse oximetry performed at the end of a 4-night period without CPAP Exclusion Criteria: * Sleep disorders other than OSA. * More than mild cardiovascular disease, unstable or recently diagnosed (within the last 6 months) cardiovascular disease such as arterial hypertension, coronary artery or cerebrovascular disease. * Any lung disease, pulmonary hypertension. * Chronic rhinitis. * Treatment with drugs that affect respiratory center drive (benzodiazepines or other sedatives or sleep inducing drugs, morphine or codeine derivates), stimulants (modafinil, methylphenidate, theophylline) * Internal, neurologic or psychiatric disease that interfere with sleep quality. * Previous intolerance to moderate or low altitude (\<2600m). * Exposure to altitudes \>1500m for \>1 day within the last 4 weeks before the study