Impact of nocturnal oxygen enrichment on high-altitude acclimatization

Nocturnal oxygen enrichment improves sleep at high altitudes but may impair acclimatization. Our purpose was to determine if nocturnal oxygen enrichment impacts acclimatization. A 7-day acclimatization protocol occurred at a field-based research site between 0 and 4,200 m. Participants were housed at 2,800 m and slept with ([Formula: see text], 32.3 ± 2.5% O₂) or without ([Formula: see text], 20.8 ± 0.1% O₂) nocturnal oxygen enrichment. Resting and steady-state cycling (5-min, 1.75 W·kg^-1) tests occurred on <i>Day 0</i> (0 m) and <i>Days 1</i>, <i>4</i>, and <i>7</i> (2,800 m). Sleep, <i>vastus lateralis</i> muscle oxygenation [oxygenated hemoglobin (O₂Hb), deoxygenated hemoglobin (HHb)], arterial blood oxygen saturation ([Formula: see text]), heart rate (HR), and expired gases were measured. Five daily hikes from 2,800 to 4,200 m were also completed. Sleep was longer (<i>P</i> = 0.028) and overnight [Formula: see text] higher (<i>P</i> < 0.001) in the [Formula: see text] (452 ± 63 min, 96 ± 1%) than the [Formula: see text] group (427 ± 63 min, 91 ± 2%). The [Formula: see text] and [Formula: see text] groups did not differ at rest in ΔO₂Hb (-1.47 ± 0.99, -1.46 ± 1.30 A.U., <i>P</i> = 0.901), ΔHHb (0.78 ± 0.84, 0.51 ± 0.96 A.U., <i>P</i> = 0.202), [Formula: see text] (93 ± 3, 93 ± 3%, <i>P</i> = 1.000), HR (59 ± 6, 64 ± 13 beats·min^-1, <i>P</i> = 0.229), respiratory exchange ratio (RER, 0.81 ± 0.07, 0.79 ± 0.06, <i>P</i> = 0.274), and ventilation body temperature pressure saturated (BTPS) (10.56 ± 2.12, 10.80 ± 1.96 L·min^-1, <i>P</i> = 0.717). The [Formula: see text] and [Formula: see text] groups also did not differ while cycling in ΔO₂Hb (-2.96 ± 3.03, -1.70 ± 3.46 A.U., <i>P</i> = 0.278), ΔHHb (7.59 ± 4.65, 6.34 ± 3.21 A.U., <i>P</i> = 0.451), [Formula: see text] (90 ± 6, 89 ± 6%, <i>P</i> = 0.875), HR (113 ± 10, 118 ± 16 beats·min^-1, <i>P</i> = 0.408), RER (0.89 ± 0.06, 0.89 ± 0.07, <i>P</i> = 0.756), and ventilation BTPS (54.00 ± 15.42, 60.18 ± 18.42 L·min^-1, <i>P</i> = 0.371). [Formula: see text] while cycling returned toward <i>Day 0</i> (0 m) values by <i>Day 7</i> (2,800 m) in both groups (<i>P</i> < 0.001) indicating short-term acclimatization. Nocturnal oxygen enrichment improves sleep but does not impair short-term acclimatization when completing daily prolonged exercise.<b>NEW & NOTEWORTHY</b> This work examined the impact of nocturnal oxygen enrichment on short-term high-altitude acclimatization to 2,800 m while completing daily hikes to 4,200 m. Recurrently dampening the required hypoxic stimulus for acclimatization via nocturnal oxygen enrichment improved sleep but did not impair short-term high-altitude acclimatization. This was evinced through ventilatory and cardiovascular adjustments that improved arterial blood oxygen saturation after 7 days.