Hypoxic depression of melatonin secretion after simulated long duration flights in man.

Fatigue is often reported after long duration flights. Mild hypobaric hypoxia caused by pressurization may be involved in this effect through disruption of circadian rhythms, independent of the number of time zones crossed. In this controlled crossover study we assessed the effects of two levels of hypoxia equivalent to 8000 and 12,000 ft on the rhythm of plasma melatonin concentrations, a marker of circadian rhythmicity. Sixteen healthy young male volunteers (23-39 years) were exposed in a hypobaric chamber for 8 hr (08:00-16:00 hours) to 8000 ft, followed 4 wk later by 12,000 ft. Plasma melatonin was assayed over two 24-hr cycles (control and hypoxic exposure) every 2 hr in all subjects. We found a significant decrease in the nocturnal melatonin peak after hypoxic exposure at both altitudes, and we found that this effect was age dependent for the 12,000-ft exposure: the decrease was only seen in the younger subjects (23-28 years). Analysis of heart rate variability allowed us to demonstrate that the older and less trained subjects (29-39 yr) in our study exhibited a far greater increase in sympathetic tone than the younger subjects during the 12,000-ft exposure. These results show that hypoxic depression of melatonin secretion may be influenced by individual factors such as age, physical fitness and sympathetic reactivity to hypoxia. Our findings suggest that hypoxia may by itself contribute at least in part to postflight fatigue after long duration flights, and to the clinical disorders of jet lag in transmeridian flights through its effects on the circadian system.