BACKGROUND: AND PURPOSE: The aim of this study was to determine how the sleep stage and body position influence the effective pressure (Peff) in standard upward titration and experimental downward titration. METHODS: This study applied successful manual titration of continuous positive airway pressure over 3 hours [including at least 15 min in supine rapid eye movement (REM) sleep] followed by consecutive downward titration for at least 1 hour to 22 patients with moderate-to-severe obstructive sleep apnea. We analyzed baseline polysomnography variables and compared the effective pressures (Peff1(upward) and Peff2(downward)) between non-REM and REM sleep and between supine and lateral positions using the paired t-test or Wilcoxon signed-rank test. RESULTS: During upward titration, Peff1 increased during REM sleep compared to non-REM sleep [9.5±2.9 vs. 8.9±2.7 cm H₂O (mean±SD), ΔPeff1(REM–non-REM)=0.6±1.1 cm H₂O; p=0.024]. During downward titration, Peff2 was higher in a supine than a lateral position (7.3±1.7 vs. 4.8±1.5 cm H₂O, ΔPeff2(supine-lateral)=2.5±1.3 cm H₂O; p=0.068). When comparing both upward and downward titration conditions, we found that Peff2 was significantly lower than Peff1 in all sleep stages, especially during REM sleep (Peff1(REM) vs. Peff2(REM)=9.5±2.9 vs. 7.4±3.3 cm H₂O) with an overall difference of 2.1±1.7 cm H₂O (p < 0.001). Peff in supine sleep decreased from 9.4±3.0 cm H₂O (Peff1(supine)) to 7.6±3.3 cm H₂O (Peff2(supine)), with an overall difference of 1.8±1.6 cm H₂O (p < 0.001). CONCLUSIONS This study has revealed that the collapsibility of the upper airway is influenced by sleep stage and body position. After achieving an initial Peff1, a lower pressure was acceptable to maintain airway patency during the rest of the sleep. The observed pressure decrease may support the use of an automated titration device that integrates real-time positional and sleep-stage factors, and the use of a lower pressure may improve fixed-pressure-related intolerance.