Ever stay up so late you feel like parts of your brain are falling asleep? They might be. In the April 28 issue of Nature, researchers used too-fun-to-resist toys to keep rats up for hours longer than usual, measuring electrical activity in the rodents’ brains with tiny implanted wires. Although the animals remained active, with most brain cells firing as erratically as they normally do in alert animals, small groups of neurons began flipping over to a sleeplike state, becoming electrically silent before firing in unison. Trained rats lost the knack for food-nabbing tricks as neurons in learning-related brain regions dozed off, perhaps explaining some of the deteriorating dexterity, flagging attention and questionable judgment seen in humans who are sleep-deprived.