Sleep Apnea Tests at Lower Altitudes May Miss the Problem, Trial Finds

Millions of people around the world live permanently at moderate altitude, generally defined as elevations above 1000 meters, where the thinner air subtly reshapes how the body breathes during sleep. For these highlanders, a routine visit to a sleep laboratory can involve an unexpected variable that most patients and even some clinicians never consider: the laboratory itself may sit hundreds of meters lower than home. A new randomized crossover trial published in the Journal of Clinical Sleep Medicine suggests that this difference is not trivial. When healthy residents of the Swiss Alps descended to a sleep laboratory at 590 meters, their nighttime oxygen levels improved and their sleep-disordered breathing measurably eased within just two nights, a finding with direct implications for how sleep apnea is diagnosed in mountain communities.

The study, led by Nadja Deflorin and Tsogyal Daniela Latshang of the Cantonal Hospital of Grisons in Chur, Switzerland, enrolled 44 healthy adults, half of them women, with a median age of 47 years. All participants lived above 1000 meters, at a median home altitude of 1230 meters, and none had spent a night below 1000 meters in the four weeks before testing. Each volunteer underwent polysomnography for two consecutive nights at home and, after a two-week washout period, two consecutive nights at the sleep laboratory in Chur at 590 meters. The order of testing was randomized with a computer-generated sequence, and the trial was registered at clinicaltrials.gov under identifier NCT05826808. Of 176 planned recordings, 174 were completed successfully, a remarkably high data completion rate for portable sleep studies.

The primary outcome was T90, the amount of time spent during sleep with blood oxygen saturation below 90 percent, a measure that previous research has linked to cardiovascular mortality. At home altitude, participants spent an average of 5 minutes per night below this threshold, compared with just 1 minute at 590 meters, a mean difference of 4 minutes that reached statistical significance. Nocturnal oxygen saturation rose from an average of 93.8 percent at moderate altitude to 95.2 percent at low altitude. More striking were the breathing metrics: the apnea-hypopnea index, which counts the number of breathing interruptions per hour, fell from 14.2 events per hour at home to 9.2 events per hour at 590 meters, while the oxygen desaturation index dropped from 10.0 to 6.0 events per hour.

The reduction in overall apnea-hypopnea index was driven almost entirely by obstructive events rather than central ones. Obstructive apneas and hypopneas, in which the upper airway collapses or narrows despite continuing respiratory effort, fell from 13.4 to 8.6 events per hour at the lower altitude, while central events showed no significant change. This pattern aligns with earlier observations that long-term high-altitude residents are prone to obstructive rather than periodic central breathing disturbances, because their physiological adaptations to chronic hypoxia, including larger lung volumes and greater tolerance of low oxygen, prolong the time it takes for falling saturation to trigger an arousal. The authors also point to anatomical factors, noting that highland populations often show larger neck circumferences and higher waist-hip ratios, potentially related to diets rich in salt, meat, and fat, all of which favor airway collapse during sleep.

Sleep architecture itself shifted only modestly with descent. Slow wave sleep, the deep restorative stage, was significantly higher on the second night at 590 meters, reaching 10 percent of total sleep time compared with 8 percent at home altitude, likely because fewer breathing disturbances meant fewer micro-arousals fragmenting the night. Transcutaneous carbon dioxide levels were slightly higher at low altitude during the first night, particularly during non-REM sleep, reflecting the reduced ventilatory drive that comes with richer inspired oxygen. Morning systolic blood pressure was also lower at 590 meters, averaging 115 millimeters of mercury compared with 120 at home, though values remained within the normal range. Questionnaires told a different story: participants rated their subjective sleep quality, sleepiness, and insomnia symptoms identically at both altitudes, unaware that their bodies were behaving differently.

One of the most clinically consequential findings concerned disease classification. At home altitude, six of the 44 participants, or 13.6 percent, crossed the threshold for severe obstructive sleep apnea with an apnea-hypopnea index above 30 events per hour. At 590 meters, only one participant, or 2.3 percent, met that criterion, a statistically significant difference. In other words, a diagnostic study performed at a lower-altitude laboratory would have labeled five people as not having severe sleep apnea who, sleeping in their own beds, clearly did. The authors conclude that sleep-disordered breathing assessments conducted at lower-than-living altitude may systematically underestimate the severity of the condition in patients who permanently reside at moderate elevation.

The trial also uncovered a pronounced sex difference. In male participants, descent to low altitude produced a larger effect on T90, with a mean difference of 7 minutes, and on REM sleep, which increased by 4 percent of total sleep time at the lower elevation. Regression analyses identified male sex as a significant predictor of both T90 and the oxygen desaturation index, with age also predicting desaturation frequency. Among males, 27.3 percent showed severe sleep apnea at home altitude versus essentially none at 590 meters, while no females crossed the severe threshold at either altitude. The researchers suggest that women may regulate breathing better at moderate altitude, consistent with prior studies at 3270 and 3800 meters showing sex-specific differences in hypoxic ventilatory response, loop gain, upper airway anatomy, and carbon dioxide reserve.

The physiological logic behind these results rests on the interplay between inspired oxygen pressure and ventilatory control. At 1230 meters, the partial pressure of inspired oxygen is low enough to destabilize breathing during sleep, particularly in REM stages when the ventilatory response to both hypoxia and hypercapnia naturally wanes and respiratory effort after airway occlusion diminishes. Descending to 590 meters raises inspired oxygen pressure, stabilizes the control system, and shortens the duration of each obstructive event because arterial saturation falls below arousal-triggering thresholds more quickly. The effect appeared within a single night, indicating that even brief exposure to lower altitude is sufficient to alter the diagnostic picture, and the two-week washout at home altitude confirmed the changes were reversible and altitude-dependent.

The authors recommend that polysomnography for patients living above 1000 meters be performed at their individual home elevations whenever possible, especially for male patients in whom the altitude effect on oxygenation, REM sleep, and apnea severity was most pronounced. They caution that their findings come from healthy volunteers and may not extend directly to patients with severe comorbidities or to those living at substantially higher elevations, and they call for further studies in people with diagnosed obstructive sleep apnea and for deeper investigation of sex differences. Still, the message for the millions of highlanders worldwide is clear: where you sleep when the sensors are attached matters, and a valley-floor clinic may paint a deceptively rosy picture of what happens in a mountain bedroom.

Subject of Research: The effect of short-term descent to low altitude on sleep-disordered breathing in healthy moderate-altitude residents

Article Title: The effect of short-term descent to low altitude in healthy residents at moderate altitude — a randomized crossover trial

Article References: Deflorin, N., Furian, M., Vontobel, J., Niederseer, D., & Latshang, T. D. (2026). The effect of short-term descent to low altitude in healthy residents at moderate altitude — a randomized crossover trial. Journal of Clinical Sleep Medicine, 22(1), Article 122. https://doi.org/10.1007/s44470-026-00128-1

Image Credits: AI Generated

DOI: 10.1007/s44470-026-00128-1

Keywords: sleep apnea, polysomnography, altitude, hypoxemia, sleep-disordered breathing, apnea-hypopnea index, oxygen saturation, REM sleep, slow wave sleep, randomized crossover trial, high-altitude residents, sex differences