When it happens: Severe-stage dementia
Why it happens: If your loved one's sleep-wake cycle doesn't seem too disrupted -- that is, he or she also sleeps at night -- this daytime sleep is likely a function of the toll that the disease is taking (along with other chronic illnesses, if they're present). It may not be sleep at all, but a resting state. Other medical causes can include depression, medication side effects, or another health issue.
What you can do:
Be sure to mention a noticeable increase in sleep to your loved one's doctor in case it's a sign of something correctible, such as a medication.
Don't just give up. It can be hard to communicate with, or know what to say to, a person who seems pretty much sacked-out and unresponsive. But your presence is felt and probably appreciated, so you don't want to leave the person alone constantly. You may also be needed.
Still, don't feel you must be a 24-hour entertainment channel, either. The resting is biologically necessary.
Stop talking when the person seems to shut down, and casually pick up talking when your loved one seems alert again.
If you're concerned about your loved one oversleeping or "zoning out" in the middle of a conversation or activity, try gently rubbing his or her hand. This gentle stimulation may be just rousing enough.