Mechanisms underlying high-altitude intolerance as well as exercise performance limitation in hypoxia still remain to be fully understood. Recent data suggest that sleep disturbances on one hand and cerebral perturbations on teh other hand may be key mechanisms. The investigators evaluated 12 healthy subjects at sea level and at 4400 m of altitude for 7 days in order to better describe sleep and cerebral responses. The investigators hypothesized that sleep and cerebral disturbances play a critical role for the developement of acute mountain sickness and for exercise performance limitation during acute high-altitude exposure.