Each of us carts around a 3-lb. universe that orchestrates everything we do: directing our conscious actions of moving, thinking and sensing, while also managing body functions we take for granted, like breathing, keeping our hearts beating and digesting our food. It makes sense that such a bustling world of activity would need rest. Which is what, for decades, doctors thought sleep was all about. Slumber was when all the intricate connections and signals involved in the business of shuttling critical brain chemicals around went off duty, taking time to recharge. We're all familiar with this restorative role of sleep for the brain--pulling an all-nighter or staying awake during a red-eye flight can not only change our mood, but also affect our ability to think clearly until, at some point, it practically shuts down on its own. When we don't get enough sleep, we're simply not ourselves.
Yet exactly what goes on in the sleeping brain has been a biological black box. Do neurons stop functioning altogether, putting up the cellular equivalent of a Do Not Disturb sign? And what if a sleeping brain is not just taking some well-deserved time off but also using the downtime to make sense of the world, by storing away memories and captured emotions? And how, precisely, is it doing that?
A year later, a biological explanation for why poor sleep might be linked to Alzheimer's emerged. Dr. Maiken Nedergaard, co-director of the Center for Translational Neuromedicine at the University of Rochester, identified a previously ignored army of cells that is called to duty during sleep in the brains of mice and acts as a massive pump for sloshing fluid into and out of the brain. This plumbing system, which she dubbed the "glymphatic system" (it works in parallel to the lymph system that drains fluid from other tissues in the body), seemed to perform a neural rinsing of the brain, swishing out the toxic proteins generated by active neurons (including those amyloid fragments) and clearing the way for another busy daily cycle of connecting and networking.
Taken together with Holtzman's discovery that levels of amyloid spiked during the day and dropped during sleep, Nedergaard's findings gave further credence to the theory that sleep might perform a housekeeping function critical for warding off diseases like Alzheimer's. "These results very much support the notion that one of the roles of sleep is to actually accelerate the clearance of beta amyloid from the brain," says Nora Volkow, director of the U.S. National Institute on Drug Abuse.