@alanbruce, There are a few reasons I can think of right off. Apneas do tend to be worse when you sleep on your back. The nights it is higher may have to do with your position. Another possibility-apnea can also be higher when you are in REM sleep. My next thought is you may experience a short period of mask leakage before you realize and readjust your mask. If the fit of your mask is off, just turning your head alittle could cause more leak. My final thought, if you are using a nasal mask (one that does not cover your mouth) you could possibly be having times you are breathing through your mouth, which causes you to not get full benefit of the PAP pessure. (JMO-from a Sleep tech in Kansas City MO)