Many neurological diseases — from Alzheimer's disease to stroke and dementia — are associated with sleep disturbances, Nedergaard notes. The study suggests that lack of sleep could have a causal role, by allowing the byproducts to build up and cause brain damage. "This could open a lot of debate for shift workers, who work during the nighttime," Nedergaard predicts. "You probably develop damage if you don't get your sleep."
One unknown, however, is whether the need to remove waste products actively regulates sleep — whether, for example, the buildup of metabolic byproducts makes us sleepy. Researchers also wonder how the fluid-filled channels change shape during sleep, and whether clearing waste actually improves the function of neurons.
Nor are other researchers certain that cleansing is sleep's sole core function. No one function of sleep necessarily rules them all, says sleep scientist Derk-Jan Dijk of the University of Surrey in the United Kingdom. "Sleep probably has many functions," he says, just as the weekend is variously for shopping, socializing, and cleaning the house.
But now that Nedergaard and her colleagues have identified this nightly brainwashing in mice, Czeisler says, scientists can investigate whether it occurs in all species, and to what extent. "One could imagine that different species have evolved different additional functions of sleep to suit their different habitats, … but this will help resolve if there is some shared function of sleep across species," he concludes.
This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org