Years ago I was doing errands for the mother of a girl I knew from high school. The older woman was in her early 70s and in failing health. After a few years, she died of congestive heart failure. My mother, about 15 years older, spends most of her time asleep lately...
I generally (selfishly) get her up if she is still sleeping after 15 hours, just so she won't be up too late that night since I have to stay up as late as she does.
Mother gets up later each day and goes to bed earlier and earlier. She only eats 2 meals a day, she's not awake long enough to eat more.
You can get mom to the drs for a physical, but aging is simply what our bodies do. I don't bounce out of bed at 6 am ready to go for the day anymore, nor do I expect that I ever will again.
88 is well past the "normal" lifespan. You can't expect mom to have the energy or desire to do the same things. Sleeping a lot? She's TIRED from living! You don't say that she has any specific ailments, but trust me, being 88 is big "ailment" for most people.