This study is a randomized double-blinded crossover pilot study. The investigators will test whether Venlafaxine has important effects on the apnea hypopnea index. Venlafaxine is a serotonin-norepinephrine reuptake inhibitor (SNRI) for the treatment of depression and anxiety. Venlafaxine increases serum serotonin level, which may affect arousal threshold. Furthermore, higher serotonin level theoretically may improve muscle tone, including upper airway muscle. Therefore, the investigators hypothesize that venlafaxine may decrease arousal threshold and improve muscle tone, leading to improvement of OSA.