National Geographic

productivity and play. Thomas Edison, who gaveus light bulbs, said that “sleep is an absurdity, abad habit.” He believed we’d eventually dispensewith it entirely.A full night’s sleep now feels as rare and old-fashioned as a handwritten letter. We all seem tocut corners, fighting insomnia through sleepingpills, guzzling cofee to slap away yawns, ignor-ing the intricate journey we’re designed to takeeach evening. On a good night, we cycle four orfive times through several stages of sleep, eachwith distinct qualities and purpose—a serpen-tine, surreal descent into an alternative world.STAGES 1-2AS WE FALL INTO SLEEP, OUR BRAIN STAYSACTIVE AND FIRES INTO ITS EDITING PROCESS—DECIDING WHICH MEMORIES TO KEEP ANDWHICH ONES TO TOSS.The initial transformation happens quickly. Thehuman body does not like to stall between states,lingering in doorways. We prefer to be in onerealm or another, awake or asleep. So we turn ofthe lights and lie in bed and shut our eyes. If ourcircadian rhythm is pegged to the flow of day-light and dark, and if the pineal gland at the baseof our brain is pumping melatonin, signaling it’snighttime, and if an array of other systems align,our neurons swiftly fall into step.Neurons, some 86 billion of them, are the cellsthat form the World Wide Web of the brain, com-municating with each other via electrical andchemical signals. When we’re fully awake, neu-rons form a jostling crowd, a cellular lightningstorm. When they fire evenly and rhythmically,expressed on an electroencephalogram, or EEG,by neat rippled lines, it indicates that the brainhas turned inward, away from the chaos of wak-ing life. At the same time, our sensory receptorsare muled, and soon we’re asleep.Scientists call this stage 1, the shallow end ofsleep. It lasts maybe five minutes. Then, ascend-ing from deep in the brain, comes a series ofelectric sparks that zap our cerebral cortex, thepleated gray matter covering the outer layer ofthe brain, home of language and consciousness.These half-second bursts, called spindles, indi-cate that we’ve entered stage 2.New memories areconsolidated duringsleep. What happens inthe brain? At the Uni-versity of Tsukuba, nearTokyo, Takeshi Sakuraistudies the questionwith optogenetics—inwhich a laser turns indi-vidual brain cells onor off in mice that aregenetically engineeredto be sensitive to it.50 NATIONAL GEOGRAPHIC