Hello and hope you are doing well.
Normal oxygen saturation values are 97% to 99% in a healthy individual. It is not normal to have low O2 sats at night. Low O2 sats while sleeping could sometimes be due to obstructive sleep apnea. When a person sleeps the airways are usually patent allowing normal passage for air entry. The upper airway that is at the region of the tongue and the soft palate is the most compliant (soft) part. So, this is liable to collapse and cause airway obstruction. I would advise you to consult a sleep specialist who would assess with first a sleep questionnaire, and then he may ask for a polysomnogram, which is an overnight sleep study as this helps to detect the apneas.
Hope this helped and do keep us posted.