Study Objectives Assess acute changes in nocturnal sleep oxygenation, sleep disordered breathing and symptoms of Acute Mountain Sickness (AMS) in healthy High Altitude (HA) sojourners. Methods Ten subjects born at Sea Level (SL) were enrolled and underwent nocturnal polygraphy for 5 consecutive nights at HA and 2 nights before and after HA (2761 m). Nocturnal oxygen profiles were characterized by the mean Oxyhaemoglobin Saturation (SpO2) during sleep, and sleep apnea severity was represented by the Apnea-Hypopnea Index (AHI). Repeated measures linear regression was used to examine responses in outcome variables (SpO2, logAHI and LLS) between HA and SL. Mean SpO2 on night 1 at HA was used to stratify post-hoc analyses based on mean level of nocturnal desaturation and logAHI. Subjects completed the Lake-Louise Score (LLS) survey every day. Results Compared to SL, mean SpO2 was lower at HA ...