This study investigates the sympathetic neural mechanisms in patients with obstructive sleep apnea (OSA) and their response to continuous positive airway pressure (CPAP) treatment. Blood pressure, heart rate, and sympathetic nerve activity were measured during wakefulness and sleep in 10 OSA patients, and in 10 age- and sex-matched control subjects, as well as in 5 obese subjects without OSA. Patients with OSA exhibited high levels of sympathetic activity even when awake (P < 0.001). During sleep, blood pressure and sympathetic activity did not decrease, and peak sympathetic activity increased during stage II sleep and rapid eye movement (REM) sleep (P < 0.001). CPAP treatment significantly reduced sympathetic activity and blood pressure during sleep (P < 0.03). The findings suggest that OSA patients have elevated sympathetic activity, which is further increased during sleep, and that CPAP treatment can attenuate these increases. These results highlight the importance of sympathetic activation in OSA and the potential benefits of CPAP in managing associated cardiovascular risks.This study investigates the sympathetic neural mechanisms in patients with obstructive sleep apnea (OSA) and their response to continuous positive airway pressure (CPAP) treatment. Blood pressure, heart rate, and sympathetic nerve activity were measured during wakefulness and sleep in 10 OSA patients, and in 10 age- and sex-matched control subjects, as well as in 5 obese subjects without OSA. Patients with OSA exhibited high levels of sympathetic activity even when awake (P < 0.001). During sleep, blood pressure and sympathetic activity did not decrease, and peak sympathetic activity increased during stage II sleep and rapid eye movement (REM) sleep (P < 0.001). CPAP treatment significantly reduced sympathetic activity and blood pressure during sleep (P < 0.03). The findings suggest that OSA patients have elevated sympathetic activity, which is further increased during sleep, and that CPAP treatment can attenuate these increases. These results highlight the importance of sympathetic activation in OSA and the potential benefits of CPAP in managing associated cardiovascular risks.