We would like to comment on the paper by Elmenhorst et al1 regarding the quality of a short nocturnal sleep episode in double aircrews on commercial ultra-long-haul flights.The study’s goal was to assess the effects of mild hypobaric hypoxia at a simulated 8000 ft altitude. The authors discovered that hypoxia had a significant impact on sleep with an increase in N2 sleep and a rise in heart rate, the effects were reversible once the hypoxia was corrected with enriched O2 air inhalation.A study limitation is the absence of any daytime hypoxic exposure during the waking period preceding the actual study when the objective was to study ultra-long-haul flights when hypoxia is present. Hypoxia can occur during the waking period of a long-distance flight; unfortunately, the authors did not plan for or discuss this though data do exist in the literature.Indeed, we documented on a circadian basis the effects of 8-hr mild hypobaric hypoxia simulating a flight in a pressurized cabin. Following this hypoxic exposure, we discovered a phase delay in the core body temperature rhythm, and changes in melatonin and cortisol circadian rhythms which could explain, at least in part, subjective complaints of poor recovery sleep quality.2–4 Last, the effects of hypoxia on sleep architecture as measured by polysomnography allowed to show an increase in sleep onset latency and sleep fragmentation, and a reduction in the total sleep period, during the two nights following the hypoxic exposure.5These findings complement and shed light on potential mechanisms for the effects of hypoxia on sleep, although the experimental design of the studies differs. To make a realistic inventory of aircrews’ sleep quality on long and ultra-long-haul flights, we believe it is necessary to factor in hypoxia’s alteration of the circadian time structure.