STUDY OBJECTIVES: Hypoxia is known to generate sleep-disordered breathing but there is a debate about the pathophysiological responses to two different types of hypoxic exposure: normobaric hypoxia (NH) and hypobaric hypoxia (HH), which have never been directly compared. Our aim was to compare sleep disorders induced by these two types of altitude. METHODS: Subjects were exposed to 26 h of simulated (NH) or real altitude (HH) corresponding to 3,450 m and a control condition (NN) in a randomized order. The sleep assessments were performed with nocturnal polysomnography (PSG) and questionnaires. Thirteen healthy trained males subjects volunteered for this study (mean +/- SD; age 34 +/- 9 y, body weight 76.2 +/- 6.8 kg, height 179.7 +/- 4.2 cm). RESULTS: Mean nocturnal oxygen saturation was further decreased during HH than in NH (81.2 +/- 3.1 versus 83.6 +/- 1.9%; P