Sleep it out
We know from personal experience that sleep is not just another brain state but a basic requirement for normal brain function while we are awake. Mental fatigue, poor decision-making, impaired learning, and a heightened risk of migraine and epileptic attacks ensue when we are sleep deprived—and chronic and complete insomnia ultimately lead to death in humans, rats, and flies alike (1). Why does normal brain function deteriorate with prolonged waking and require sleep to be restored? On page 373 in this issue, Xie et al. (2) report that during sleep, waste products of brain metabolism are removed from the interstitial space among brain cells where they accumulate. Sleep, therefore, might be required for potentially toxic metabolites—the very results of a working brain—to be cleared from the tissue.