Low oxygen level during Sleeping.

I have asthma problems from more than 8 years. Doctors have given me budesonide & formoterol inhaler. My problem is a little bit less now as compared to before. I have to take the inhaler sometimes and mostly during winters. It has been 15+ days since i have recovered from COVID but whenever i sleep with fan at much speed and no blanket i get breathing issues due to cold and it happens during summer too. I always have to cover myself in blanket whenever i sleep. Yesterday i had breathing problem and chest tightness at around 3am, i checked my oxygen level and it was 89, but when i switched off the fan and took the blanket my chest tightness was relieved and SpO2 came back above 93. During the day my spo2 is above 95. I became worried as to why was my spo2 so low during night. It always happened to me during night as a child it's just that this time i had spo2 monitor to check it. I didn't have spo2 monitor as a child maybe it was since the starting. Plz tell me is it normal in asthma?