OBJECTIVES: Effective nasal continuous positive airway pressure (CPAP) therapy reduces the cardiovascular outcomes associated with obstructive sleep apnea (OSA), but the mechanism behind this effect is unclear. We investigated if OSA patients during wakefulness showed signs of increased sympathetic activity and decreased vasoreactivity in cerebral cortical vessels as measured with near-infrared spectroscopy (NIRS), and if this may be reversed by CPAP treatment. SUBJECTS AND METHODS: 23 OSA patients (mean age, 55y) naive to CPAP were included in a prospective interventional study. The OSA patients received CPAP therapy for at least two months. Cortical low-frequency oscillation (LFO) amplitudes and vasoreactivity during a breath hold test were measured with NIRS and were compared between baseline and after CPAP treatment. Baseline values also were compared to 13 healthy controls (mean age, 52y). RESULTS: We found a decrease in LFO amplitudes after CPAP therapy (P=0.022) in OSA patients. We found no differences in LFO amplitudes between OSA patients and healthy controls (P=0.934). There were no differences in peak vascular response following breath hold tests in OSA patients before and after CPAP therapy (P=0.158) or compared to healthy controls (P=0.740). CONCLUSION: Our NIRS study revealed a decrease in LFO amplitude following two months of CPAP treatment in OSA patients, which may reflect a decrease in sympathetic activity affecting cortical vessels.