It is one of the main reasons for high RBC. Most bodybuilders who struggle with blood viscosity blame it on the gear, but many suffer from undiagnosed sleep apnea or they have been diagnosed, but leave it untreated. They hate the mask. I don't like it either, as I am a stomach sleeper, but I still wear it. I figured it out.
If I don't wear it, because I had a head cold, I will wake up with a headache and feel like total crap for the rest of the day. I have no idea how guys can go without theirs. Horrible feeling.