It is not unusual that people with Alzheimer's Disease develop a markedly variable sleep schedule. This is a disease that causes a loss of brain cells and interferes with communication
between many of the remaining cells. The control center for sleep is located in the middle of the brain, and Alzheimer's Disease can wreak havoc in that center, which can lead to the kind of problem your mom is having.
It is important for your mom to be evaluated by her physician for her problem with intermittent excess sleeping. In general, people with Alzheimer's Disease feel better and think more clearly if they are on a more regular sleep schedule. It may be she has some other medical problem, a psychiatric problem or is on a medication that is playing a role in this. It may also be she will need a medication to better regulate her sleeping. In most cases, this is a fixable problem, and such a fix will likely improve her energy, mood and concentration.