Fluctuations in autonomic cardiovascular regulation during exposure to high altitude may increase the risk of heart attack during waking and sleep. This study compared heart rate variability (HVR) and its components during sleep at low altitude and after 30-41 hours of acclimatization at high altitude (3480 m) in five mountain marathon runners controlled for diet, drugs, light-dark cycle and jet lag. In comparison to sea level, RR-intervals during sleep at high altitude decreased significantly (P < 0.001). The significant increase in sympathetic autonomic cardiovascular modulation at high altitude protects against excessive oxygen deprivation during sleep. Increases in R-R intervals can require longer periods of acclimatization at 3480 m to mitigate the effects of altitude/hypoxia on sympathetic tone, thus reducing cardiovascular distress at rest during waking and sleep and probably before during and after athletic performance at altitude.

Altered blood chemistry, acid-base and electrolyte are suggested determinants of sleep disturbance, with frequent arousal at high altitude even in well and long-trained altitude marathon runners. In this sample of experienced altitude marathon runners with maximal aerobic power at sea level of 61.4 ± 2.7 ml/kg −1 ·min −1 we found that pO 2 and percent of oxygen saturation (%SO 2 ) were lower at 2050 m and 3480 m than at sea level; pO 2 was higher after 38 -41 hours than after 30 -31 hours of acclimatization at 3480 m (P < 0.05). After ascentto 3480 m %SO 2 decreased (P < 0.003). Compared to sea level values, pH increased at high altitude (P < 0.05) consistent with changes in pCO 2 and (P < 0.05). Nocturnal %SpaO 2 at a sleeping altitude of 3480 m was lower (P < 0.05) than at sea level. At high altitude, the percent of wake (W) time and delay falling asleep (DFA) increased, whereas non-rapid eye movement sleep (N-REM), REM sleep and total sleep time (TST) decreased (P < 0.05). Simple regression analysis disclosed a significant correlation between the changes in TST and the percent of REM sleep and the changes in %SpaO 2 recorded during sleep (P < 0.05). Simple regression analysis showed a positive correlation between the changes in pO 2 at higher altitude and the percent of W and of TST (P < 0.05). The changes in pO 2 , tCO 2 and . Simple regression analysis demonstrated that the changes in pH at high altitude correlated positively and significantly with the percent of W and the DFA and negatively with the percent of changes in NREM sleep, REM sleep, NREM + REM sleep (P < 0.05). The decrease in the TST at high altitude correlated significantly and negatively with the changes in pCO 2 , tCO 2 , and [K + ] (P < 0.05). Our data demonstrate that the arterialized ear lobe techniques we used for evaluating most of the changes in blood chemistry, acid-base, electrolyte and blood lactate metabolism aresuitable for clinical and laboratory assessment and are important predictors of the quality and quantity of acclimatization and sleep at high altitude.