Six men, normally working shifts of 7 days at high altitude (HA, 3800 m, ∼480 mm Hg barometric pressure) followed by 7 days of rest at sea level (SL), were studied during the last days of their HA and SL shifts with a 24-h constant routine protocol of sustained wakefulness and minimal activity. The amplitude of the circadian oscillations of oxygen consumption, breathing rate, thoracic skin temperature and diastolic pressure did not differ between HA and SL. At HA, the amplitude of the tympanic and calf temperature oscillations, were, respectively, lower and higher than at SL. End-tidal PCO2 and systolic pressure had larger amplitude oscillations at HA than at SL. Hence, also in humans, as previously shown in animals, hypoxia can affect some circadian patterns, including those involved in thermoregulation. These effects of hypoxia could contribute to sleep disturbances at HA and in patients with cardiorespiratory diseases. Copyright © 2001 Elsevier Science B.V.