Recently sham CPAP was developed, which mimicked all the characteristics of a true CPAP except for the null pressure applied to the upper airway of the patient. Moreover, sham CPAP doesn't influence sleep efficiency, arousals, or apnea-hypopnea index. Therefore, in this project, using sham CPAP as a placebo, we propose a randomized double-blind placebo controlled trial to assess the effectiveness of CPAP in severe OSA patients without daytime sleepiness. We also establish a model to predict the CPAP effect. The results would change the current practice of managing severe OSA patients without daytime sleepiness.