One angle not touched on here is the link between Alzheimer's disease (AD) and sleep. There has always been evidence that AD patients sleep less than the general population, but obviously there's a chicken and egg problem here - are they sleeping less because of neuronal disruption caused by the disease, or is the disease progression catalyzed by a lack of sleep[1-3]? While I think this is still an open question, there's growing research that interaction with sleep related systems in the brain can have a major therapeutic impact on the disease itself - obviously this doesn't actually answer if it's 'sleep' or some process related to sleep, but clearly something is going on here [4].

From the press release associated with [4];

The new research, in mice, demonstrates that eliminating the protein – called orexin – made mice sleep for longer periods of time and strongly slowed the production of brain plaques.

“This indicates we should be looking hard at orexin as a potential target for preventing Alzheimer’s disease,” said senior author David M. Holtzman, MD, head of the Department of Neurology. “Blocking orexin to increase sleep in patients with sleep abnormalities, or perhaps even to improve sleep efficiency in healthy people, may be a way to reduce the risk of Alzheimer’s. This is important to explore further.” - [5]

[1] Xie, L., Kang, H., Xu, Q., Chen, M. J., Liao, Y., Thiyagarajan, M., … Nedergaard, M. (2013). Sleep drives metabolite clearance from the adult brain. Science, 342(6156), 373–377. http://www.sciencemag.org/content/342/6156/373

[2] Ju, Y.-E. S., McLeland, J. S., Toedebusch, C. D., Xiong, C., Fagan, A. M., Duntley, S. P., … Holtzman, D. M. (2013). Sleep quality and preclinical Alzheimer disease. JAMA Neurology, 70(5), 587–593. http://www.ncbi.nlm.nih.gov/pubmed/23479184

[3] Ju, Y.-E. S., Lucey, B. P., & Holtzman, D. M. (2013). Sleep and Alzheimer disease pathology - a bidirectional relationship. Nature Reviews. Neurology, 10(2), 115–119. http://www.nature.com/nrneurol/journal/v10/n2/abs/nrneurol.2...

[4] Roh, J. H., Jiang, H., Finn, M. B., Stewart, F. R., Mahan, T. E., Cirrito, J. R., … Holtzman, D. M. (2014). Potential role of orexin and sleep modulation in the pathogenesis of Alzheimer’s disease. The Journal of Experimental Medicine, 211(13), 2487–2496. http://jem.rupress.org/content/211/13/2487

[5] https://news.wustl.edu/news/Pages/27721.aspx