88% is the *magic* number. Depending on the brand/model Oximeter you have, that big dip may just be *noise*.
Using the software that comes with the Oximeter, there is a numeric value for "minutes under 88%" - I am considered 'not great' and I average 2-5 minutes per night under 88%. If you are consistently under 90% time to see a Doc. Lower numbers at night *are* normal. My nightly average bounces between 89-91% (I have lung scarring due to multiple PEs) so I am worse than average.
*I* am not a DOCTOR or any type of Health Care Professional. My thoughts/suggestions/ideas are strictly only my opinions.
"Only two defining forces have ever offered to die for you. Jesus Christ and the American Soldier. One died for your Soul, the other for your Freedom."
Ill add my 2¢ to the already good advice you see from Peter_c. I suspect the big dip is when the device was dislodged briefly since it's not likely your blood/O2 saturation could dive that rapidly.