My mom fell back in October. She went to the hospital, had her hip repaired, went to rehab. She never was on any meds in her life. When she was in the rehab, she developed a urinary tract infection. I kept telling them but they told me she didn't. I took her out after 4 days and put her in the hospital with 104 temperature and she became septic from it. So in and out of new rehabs she has lost her ability to really get around on her own. She lives with me, and I give her 24 hour care. She's very well hydrated. She drinks all day and eats very well and is very healthy. She's back on no meds except for a baby aspirin and some B12 but she is still not able to walk around in her room unassisted. We do some therapy but other than that she's fine except for sleeping till 3 and 4 in the afternoon. She does go to bed at 10 or 11 at night, just wondering if letting her sleep as much as she wants is okay.
Boredom/depression is certainly not the only cause of sleeping a long time but it would be fairly easy to experiment with adding activities to see if that helps. Perhaps the local theater has a "senior price" special for their 1:00 showings. Maybe the senior center has card games in the morning and then a hot lunch. Perhaps you can arrange for a visitor to come for a morning coffee break now and then. If Mom is able to get up for these things, that may give you your answer.
If she continues to sleep 16+ hours a day, consult a sleep specialist. It may be that she just needs the extra sleep after all she's been through, but the sleep clinic can help determine is there is something more than that going on.
Does she use a wheelchair sometimes? My husband really perked up once he could scoot himself around with his feet. He no longer feared falling. He also walked enough to maintain leg strength, but the wheelchair gave him a new outlook on life.