All night sleep EEG, EKG, and respiration were recorded from six young men during 2 nights at sea level and 4 nonconsecutive nights at high altitude (14.110 ft., 4301 m). Sleep at high altitude was chraacterized initially by a significant decrease in Stages 3 and 4, a significant increase in the number of arousals, and a trend towards more time spent awake. In terms of actual time spent sleep, however, a relatively good night's sleep was obtained, which suggests that the objective sleep disturbance was not commensurate with the marked subjective complaints of sleeplessness. Periodic respiration during sleep was frequent at high altitude, was quickly terminated by oxygen administration, was not clearly related to the increased number of arousals, and usually was not seen during REM periods. Heart rate was increased during sleep at high altitude. All measures tended to return towards sea level means during 12 days at altitude. We suggest that the marked increase in the number of arousals may account for the disparity between the subjective reports and objective measures of sleep disturbance at high altitude. Although the objective sleep disruption is probably related in some fashion to hypoxemia, it is unclear whether hypoxemia itself or the alkalosis commonly present shortly after arrival at altitude is the major factor.