Mom's 90 and in good health but she sleeps all night and whenever she sits down, all day.

Is this new? How long has it been going on? Have you discussed it with her doctor?

Maybe she has narcolepsy. So, she sleeps 24 hours a day? Does she do anything else? If she is able, take her for a walk every day for about 20 minutes. Have her breathe deeply. She needs more oxygen to the brain. If she sleeps well after the walk, then wakes up refreshed, she needs more activity.
Newton's Law: a body in motion tends to stay in motion. Within reason for a 90 year old. Make sure she gets plenty of water, too. Could be dehydrated:) xo