Nocturnal oxygen enrichment improves sleep at high-altitude but may impair acclimatization. PURPOSE: Determine if nocturnal oxygen enrichment impacts acclimatization. METHODS: A 7-day acclimatization protocol occurred at a field-based research site between 0-4200m. Participants were housed at 2800m and slept with (O2+, 32.3±2.5% O2) or without (O2-, 20.8±0.1% O2) nocturnal oxygen enrichment. Resting and steady-state cycling (5-min, 1.75 W·kg-1) tests occurred on Day 0 (0m) and Days 1, 4, and 7 (2800m). Sleep, textlessemtextgreatervastus lateralistextless/emtextgreater muscle oxygenation (oxygenated hemoglobin [O2Hb], deoxygenated hemoglobin [HHb]), arterial blood oxygen saturation (SPO2), heart rate (HR), and expired gases were measured. Five daily hikes from 2800-4200m were also completed. RESULTS: Sleep was longer (p=0.028) and overnight SPO2 higher (ptextless0.001) in the O2+ (452±63 min, 96±1%) than the O2- group (427±63 min, 91±2%). The O2+ and O2- groups did not differ at rest in ∆O2Hb (-1.47±0.99, -1.46±1.30 A.U., p=0.901), ∆HHb (0.78±0.84, 0.51±0.96 A.U., p=0.202), SPO2 (93±3, 93±3%, p=1.000), HR (59±6, 64±13 beats·minute-1, p=0.229), respiratory exchange ratio (RER, 0.81±0.07, 0.79±0.06, p=0.274), and ventilation BTPS (10.56±2.12, 10.80±1.96 L·minute-1, p=0.717). The O2+ and O2- groups also did not differ while cycling in ∆O2Hb (-2.96±3.03, -1.70±3.46 A.U., p=0.278), ∆HHb (7.59±4.65, 6.34±3.21 A.U., p=0.451), SPO2 (90±6, 89±6%, p=0.875), HR (113±10, 118±16 beats·minute-1, p=0.408), RER (0.89±0.06, 0.89±0.07, p=0.756), and ventilation BTPS (54.00±15.42, 60.18±18.42 L·minute-1, p=0.371). SPO2 while cycling returned towards Day 0 (0m) values by Day 7 (2800m) in both groups (ptextless0.001) indicating short-term acclimatization. CONCLUSION: Nocturnal oxygen enrichment improves sleep but does not impair short-term acclimatization when completing daily prolonged exercise.