What Causes Excessive Sleepiness In An Elderly Person?

My mother at 91 has days she cannot resist the urge to sleep most of the day. She will rise and have breakfast, then make plans for the day, but is overcome by sleepiness and may spend until noon in her chair asleep. She says she just can t seem to wake up. She will eat lunch, and eats well, then return to her chair to sleep. She sleeps well at night. She has a pacemaker and takes a blood thinner and several other drugs common to old age. We have been to the local clinic and had blood work and urinalysis done. They revealed no cause for the sleepiness. Some days she is fine. Although somewhat forgetful, her mental abilities are quite good. She may spend the day sleeping between meals and then go to bed early to sleep all night.