Patients with Alzheimer's disease often have poor nocturnal sleep quality and daytime sleepiness. In fact, the sleep difficulties at night are a common reason that Alzheimer sufferers are put in nursing homes. Caregivers are unable to stay up at night to ensure their loved one's safety, and then function in the daytime. This study showed that there is a relationship between individuals' sleep quality and Alzheimer disease markers in spinal fluid. I don't full access to the article, so can only comment on the abstract.
This study has some limitations. Participants sleep quality was assessed only via questionnaire, not by objective sleep testing. So it's unclear what is contributing to the reduced sleep quality. Also, the participants were already at risk for Alzheimer disease, having a family history of it. From this study, we can't conclude that poor sleep quality leads to Alzheimer disease. And it's not clear if improving sleep quality would lead to a decrease in the biomarkers for Alzheimer disease.