Not understanding the putting mask on during the first night of study. The first night is to see how many you have. You might have, like me, 31 in one hour but then only 3 or 4 another hour. They did not put a mask on me the first night at all. They just noted how may episodes I had and the I went back to wear the mask. The doctor had to read the results to get final count to know what to set my machine to. I guess they are all different.