Sleep at high altitude has so far been studied mainly in people in the developed world. The present study aimed to evaluate sleep architecture at 3500 m on eight healthy but unacclimatized Indian lowlanders in order to investigate whether the decrease in the blood saturation of peripheral oxygen (SpO2) and end tidal carbon dioxide (EtCO2) during their sleep at a high altitude was related to changes in sleep architecture as well as the relationship of periods of apnea with episodes of awakening. The sleep architecture on day 2 of the ascent to 3500 m revealed an increase in the number of awakenings and sleep latency and a decline in sleep efficiency, a decrease in stages S3 and S4 of NREM sleep, an increase in stage S2, no change in REM, no periodic breathing and no acute mountain sickness. The number of position changes, heart rate and apnea counts increased while SpO2 and EtCO2 decreased during sleep at 3500 m. There was a significant negative correlation of mean SpO2 and mean EtCO2 with S1 and S2, a negative correlation of EtCO2 with apnea counts and a negative correlation of mean SpO2 and EtCO2 with the participants' heart rate, and a positive correlation of apnea counts with heart rate but no relationship between the apnea counts and number of awakenings. The participants' subjective evaluation of sleep quality was that this was poor. The study revealed that both hypoxia and hypocapnia at 3500 m increased “light” sleep, hypocapnia increased apnea counts but respiratory disturbance did not appear to be of sufficient magnitude to elicit awakening on day 2 of ascent to 3500 m in the Indian lowlander participants.