At 91 your mother is tired, she has lived a very long life. My parents are both in their 90's and brag that they only sleep 6 hours at night... of course they don't count the long nap after breakfast, again after lunch, and again after dinner. Every time I pop into their home, I need to wake them up :)
many older people sleep a lot. However, you may want to talk to your moms doctor to see how she is doing physically. I always said to my moms doctor that I wanted to know when her body was shitting down so I could prepare myself. I am not saying that you are there but a frank discussion with the doctor could help if you want to know. If you don't want to know that is ok too. You do what is best for you. My dad would nap many times each day when he and my mom lived together. Now that he lives with me he only naps once each day and some days he does not nap. I think he napped out of boredom. There is much more going on at my house.
Be sure to get your mom's thyroid checked if she hasn't had that done in a while. Also her B12 and Vitamin D levels. My mom is 95 and her thyroid stopped working at about 92. She's been on thyroid medication ever since and it's given her a lot more energy. So maybe a general blood work panel is in order, just to make sure her blood is OK.
My mom also complains about not sleeping enough at night, and she does nap some during the day, but she's awake a lot of the time.
My mother is 90 and often complains that she doesn't sleep at night. She leaves the tv on so loud that we can hear it on the other side of the house, but when I check on her, sometimes she is sound asleep and sometimes wide awake. When she is in the living room with me, she often falls asleep, but doesn't know it.
My mom also complains about not sleeping enough at night, and she does nap some during the day, but she's awake a lot of the time.