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140 million people live above 2,400m worldwide and even more are exposed to high altitude each year. High altitude has been reported to cause decrements in sleep including reduced total sleep time, reduced sleep efficiency, and reduced REM sleep. Because of the disruptive effects of caffeine on sleep in normoxic conditions, there is a negative stigma towards caffeine at high altitude. Caffeine has been studied extensively in normoxic conditions, however, to our knowledge the effects of caffeine on sleep at high altitude have yet to be examined. Therefore, the aim of the current study is to examine the effects of caffeine on sleep at high altitude. A single blind, non-randomized, between subjects study was performed on 25 participants (including six females), aged 28.6 ± 9.7 (mean±SD) at 4,300 m on the Mt. Everest trekking route (Nepal). To control for withdrawal effects of caffeine, participants that self- reported consuming less than 47 mg of caffeine per day received the placebo while those that consumed more than that received the caffeine pill. Participants ingested the study drug (placebo or 200 mg of caffeine) 1.5 hours after awakening and refrained from ingesting other caffeinated drinks and food. A wireless sleep recording device was used to quantify sleep quality the following night. Caffeine significantly decreased self-selected total sleep duration (p<0.05), however other sleep metrics associated with sleep quality [sleep onset latency, wakefulness after sleep onset, light sleep (combined stages 1 and 2), deep sleep and REM] were not significantly different between placebo and caffeine. Based on this data, when sleep is already disrupted due to hypoxic conditions, caffeine does not have an impact on sleep quality when taken in the morning. However, the total sleep significantly decreased, which could potentially cause cognitive impairments the following day. NIH Build EXITOUniversity of Alaska Anchorage