Is twelve hours of sleep a night too much for a woman (94)?

My mother is ninety-four and in very good health. She takes no medications, walks without a can or walker, and participates in the exercise sessions at the assisted living facility where she has resided for over two years. She prides herself on her good health, and often states, "I go to bed at nine, and I get up at nine." I see no problem with this, but my sister, who lives several thousand miles away and has seen Mom only once in the past year-and-a half, thinks this is too much sleep, and believes Mom must have hypothyroidism. Her doctor, however, at her last check-up in June, found no symptoms of such, and did not prescribe medication. I've tried to find research on the subject, but could only find articles on insomnia in the elderly. Does anyone have any insight? Is twelve hours a night too much? Thank you!