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The objective was to examine whether abnormal breathing during sleep may affect regulation of ventilation after awakening in patients with obstructive sleep apnoea (OSAS). In 19 patients with OSA and 12 normal subjects we examined ventilatory responses to hypoxia (HVR) and to hypercapnia (HCVR) before and after sleep (BS and AS), and compared the changes in ventilatory responses with respiratory events during sleep. In the OSA group, the values of resting ventilation were significantly smaller in AS than those in BS and end-tidal partial pressure of CO2 in arterial blood (P CO2 ) (P ET CO2 ) rose significantly from BS to AS. The slopes of the HVR or HCVR did not differ between BS and AS. However, both the response lines shifted downward and minute ventilation (VE )80 (VE at arterial oxygen saturation (S aO2 ) of 80%) in HVR and VE 60 (VE at P ET CO2 of 60 mmHg) in HCVR decreased significantly from BS to AS. The percentage changes of VE 80 and VE 60 were significantly correlated with mean S aO2 , total sleep time below S aO2 of 90% and lowest S aO2 during sleep. However, in normal subjects we observed no circadian variation in their ventilatory responses. These data support the hypothesis that repeated episodes of nocturnal hypoxia and hypercapnia may modify the regulation of ventilation after awakening in patients with OSA. [ABSTRACT FROM AUTHOR]