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The hypnogram obtained in a subject during a night at 490 m (top panel) shows a normal distribution of sleep stages and several NREM/REM sleep cycles. In contrast, the hypnogram recorded during the 1st night at 4559 m (middle panel) reveals predominantly superficial sleep stages with frequent awakenings, very rare deep sleep stages 3 and 4, and no REM sleep. The hypnogram from the 3rd night at 4559 m (bottom panel) reveals a partial restoration of normal sleep architecture.
