I went through a sleep study, which is what you need to do, my oxygen went to 70s for short period but main was 83for periods of time. The doc sent me to pulmonaligist where I was diagnosed with severe emphysema.
You should have your doc set up a sleep study and go from there.
Good luck and God bless
Could be sleep apnea which can be dangerous you need to see your doctor and get evaluated through a sleep study to see if you need a c pap machine to sleep with at night to keep your O2 levels up go see your doctor and ask if he thinks you may have sleep apnea
One possible cause would be sleep apnea. You should discus this with your dr and together decide if a sleep study would be beneficial.