Objective: Obstructive sleep apnea events are more common in REM sleep, although there is no relationship between sleep phase and pharyngeal airway status. We studied the patency of the nasal airway during REM and non-REM sleep with the use of acoustic rhinometry. Methods: Serial acoustic rhinometric assessment of nasal cross-sectional area was performed in 10 subjects, before sleep and during REM and non-REM sleep. All measurements were standardized to a decongested baseline with mean congestion factor (MCF). Results: MCF in the seated position was 10.6% (±3.7) and increased with supine positioning to 16.2% (±2.3). In REM sleep, MCF was highest, at 22.3% (±1.7). In non-REM sleep, MCF was lowest, at 2.3% (±3.1). All interstage comparisons were statistically significant on repeated measures ANOVA (P < 0.05). Conclusion: REM sleep is characterized by significant nasal congestion; non-REM sleep, by profound decongestion. This phenomenon may be attributable to REM-dependent variation in cerebral blood flow that affects nasal congestion via the internal carotid system. REM-induced nasal congestion, an indirect effect of augmented cerebral perfusion, may contribute to the higher frequency of obstructive events in REM sleep.