When you were diagnosed with sleep apnea
, you had your sleep study. As you know, they kept track of your blood oxygen level, but they also measured many other values: pulse, blood pressure, number of apneic episodes per minute, amount of REM sleep, etc. The purpose of the CPAP machine is to overcome the obstruction caused by your airways and allow you to keep breathing at a regular rate throughout the night. In theory, your oxygen levels may be improved by your CPAP machine, and measuring your levels at night certainly has no downside. However, you may have had fairly normal oxygen levels prior to starting your CPAP therapy. Your mask may be improving your sleep in different ways that you wouldn't be able to measure at home (such as cutting down on the amount of apneic episodes or providing you with greater amounts of uninterrupted REM cycle sleep). The best way to determine the effectiveness of your therapy is to return to the sleep center that first evaluated you and ask them to perform a second sleep study, this time with you CPAP machine on. This is the most effective way to ensure that you are getting the therapy you need.