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...
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.
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.