This article from the Sleep Journal discusses a study to see if sleep complaints are associated with declining memory in the elderly. Researchers asked older participants how they slept, if they were sleepy in the daytime, and what medications they took, and followed them over an 8 year period.
Results showed that there was a positive association between self-reported sleepiness and memory decline. This was the case even after adjustment for the study center location, gender, age, educational level, baseline memory, insomnia severity, and prescribed sleep medicine.
The authors speculate that excess sleepiness may result from early stage brain lesions in those with cognitive decline. They also mentioned that obstructive sleep apnea (OSA) may contribute. The authors did not perform sleep studies, so some of the participants could be sleepy solely from untreated OSA.