Inclusion Criteria:  Healthy males or females Age 18 to 35 years of age Body-mass index (BMI) ≤ 27 At least two weeks after full vaccination for Sars-CoV-2. Proof of vaccination will be determined by the investigators.  Exclusion Criteria:  Current smoker or significant past smoking history (≥ 10 pack-years) History of COPD, interstitial lung disease or other pulmonary disease that in the opinion of the investigators would compromise the subject's safety at altitude History of obstructive sleep apnea, or other history of sleep disordered breathing, or previous use of nighttime continuous positive airway pressure (CPAP) Any history of chronic opioid use or prior abuse history Any history of alcohol abuse Use of any opioid substance (prescribed or otherwise) within 2-weeks of study enrollment Any medications that might, in the opinion of the investigators, interfere with or alter respiratory control, (e.g. acetazolamide). Use of oral contraceptive medications would be allowed as long as the study subject is on the same agent/dose during both arms of the study. Recent (within 3 weeks) exposure to altitudes ≥ 3,500 m at time of 1st altitude trip