Sleep is a biological imperative for most animals. Even brief periods without sleep cause cognitive impairment, and long-term continuous sleep deprivation is a road through insanity that ends in death. Research reported in Science suggests that the restorative quality of sleep may be a result of an enhanced ability to eliminate potentially neurotoxic waste products in the unconscious brain (2013, DOI: 10.1126/science.1241224). Lulu Xie and Maiken Nedergaard of the University of Rochester Medical Center determined via electrochemical methods that a sleeping mouse’s brain is 23% intercellular space by volume. That value is just 14% in an awake mouse. The team observed that fluorescent tracers penetrated 20 times as far into the brains of sleeping mice as they did into those of mice that were awake. They also showed that the sleeping brains cleared amyloid-β, a protein implicated in Alzheimer’s disease, twice as fast as awake brains. The researchers propose that the increase in intercellular volume during sleep eases fluid flow through the brain to help flush out metabolic waste products that accumulate during wakefulness.