Direct measurements were made of the pulmonary function and sleep stages in two men newly arrived at the South Pole Station. Subjective ratings of "poor" sleep were quantitated in sleep records that exhibited complete loss of stages 3 and 4 for all nights recorded. Subject S-50, who suffered the symptoms of acute mountain sickness, had further alterations in sleep with a 100% increase in stage 1 and a 50% decrease in the rapid eye movement stage. Both subjects had high levels of ventilation associated with low arterial oxygen pressures of 47 mm Hg and 51 mm Hg. Subject S-50 also had greater difficulty with respiration with a ventilatory equivalent of 60. The hypothesis is offered that the striking curtailment of stages 3 and 4 during sleep may be a normal physiological adaptation to the decreased levels of arterial oxygen pressure at the South Pole.