Nasal obstruction is a cause of disordered breathing during sleep. Our previous study demonstrated diminished end-tidal PCO2 with nose obstruction while subjects were awake. If this is also the cause during sleep, decreased CO2 stimulus may easily induce apnea, hypopnea, and disordered breathing. To test this hypothesis, six male volunteers were examined to compare sleep disorders during both nose-open and nose-obstructed conditions. End-tidal PCO2 during nose-obstructed sleep was lower than that during nose-open sleep in all of the subjects. Furthermore apnea during nasal obstruction occurred most frequently shortly after transition to a deeper sleep stage. These results suggest that diminished PCO2 stimulus combined with depressed behavioral activity play an important role for disordered breathing in nose-obstructed sleep.