Doubt it is a heart issue, could be lungs or an obstruction (sleep apnea). Do you smoke, are you overweight? Asked respectively to the first two points.
Obstructive sleep apnea generally shows low O2 levels when lying on one's back, and not when up and about, even when lying on one side or stomach.
An O2 level below 95 is considered low, but not dangerously low if above 90 I believe. 89? same as 90 given some tolerance in reading error.
I have been diagnosed with mild obstructive sleep apnea, it is mostly gone with the loss of 10 pounds, I am working on losing 25 pounds for good measure. I was (am) just overweight, not real heavy. Now about 225 at 6' 5" - 220 would be better, after that I need to just firm up with exercise.
Merry Christmas