Abstract
Brain electrical activity differs markedly between wakefulness and sleep. Concomitant shifts in the ion composition of brain extracellular fluids were thought to be a consequence rather than a cause of the sleep-wake–dependent changes in neuronal activity. On page 550 of this issue, Ding et al. (1) report the surprising observation that ionic changes in the extracellular fluid are a potent control of sleep-wake–dependent neuronal activity.