What causes high tidal/ventilation variance?

[YouTakeMyBreathAway]

My CPAP has improved my daytime sleepiness a little, but not completely. I found that my oxygen still drops to ~90% throughout the night even with CPAP.

Recently I experimented with taking Afrin and found out it improved my oxygen saturation and also decreased spikes and dips in my tidal volume / stabilized my tidal volume.

This made me wonder: What is causing my tidal volume to continuously fluctuate throughout the night? Is this is what is causing my daytime sleepiness?

Example OSCAR here (green line is when I took the Afrin): https://imgur.com/a/j2FhrUV

[Jay51]

Hello YouTakeMyBreathAway.  Recently I experimented with trying to nap lying flat on my right or left side (and flat once).  I usually sleep on a 45 degree angle upper body wedge with a tall, thick, fairly rigid SSC (it basically immobilizes my neck, chin, and airway at night).  I still used the SCC with the naps, but found my nose immediately encountered problems.  It seems like fluid from gravity immediately pooled in nose and sinus cavities (either blood and/or sinus fluid) and made breathing out of my nose almost impossible.  Upon rising to the 45 degree angle, my nasal cavities cleared up completely. 
Afrin is a decongestant and also treats nosebleeds.  It may be stopping the sinus fluid build up and/or blood from accumulating and pooling; therefore keeping the nasal passages open better.  
Just my guess.  I may be wrong.  The experts here may weigh in on this one later.