Ventilation is under metabolic as well as under behavioural control. This causes a complex interaction between states of 'vigilance' and respiration. This paper briefly summarizes sleep-related changes of respiration and presents an experimental study on the course of respiratory CO2-sensitivity during a whole night's sleep in ten healthy volunteers. The feedback control of breathing was challenged by continuous step changes of inspired CO2 every 7 min, resulting in 60, 3-step steady-state hypercapnic ventilatory responses (HCVR) per night in each subject. We analysed the variability of baseline ventilation and the effects of hypercapnia on ventilation with respect to sleep stages. There were only small differences in baseline PCO2 and ventilation between sleep stages, but a high variability of the slope of the CO2-response curves in the course of the night, ranging from 0.5 to 3.0 L min(-1) Torr(-1). The HCVR was significantly lower during REM sleep than during all stages of NREM sleep. Due to a compensatory left shift of the flattened CO2-response curves, however, ventilation at baseline CO2 as well as during slight hypercapnia varied much less than would be expected from the high variability of slopes. We conclude that the characteristics of the CO2-sensitive feedback control system of respiration, are highly variable during sleep, but due to offsetting effects, PCO2 and ventilation remain quite stable in the physiological range.