Neuroscientist Jan Born is quietly jealous of his eight-month-old daughter. “She sleeps when she wants,” he says. Then again, he says, sleep is a crucial time for learning, and she probably has more to learn about the world than the average adult. “I think about whether she needs this sleep because her hippocampus is full,” he says. The hippocampus is a node in the brain's memory network, the place memories are first encoded for transferral later to longer-term storage. Sleep is one way its contents are downloaded to other regions of the brain where it is thought they are interpreted and stored. “We know that during sleep the brain processes a wide range of memory types,” says Robert Stickgold, a neuroscientist at Beth Israel Deaconess Medical Centre in Boston, Massachusetts. Researchers know that a bit of shut-eye helps you recall all manner of things, from newly acquired motor skills, such as how to play the piano, to what you wore to the theatre last night. But sleep is not a passive storage process, like saving a video file to a hard drive. Sleep also reconfigures memory. It helps us edit the files — adding or removing content or emotional tone, for example — and re-save them. “This isn't just memory representation getting stronger,” says Born, who studies sleep and memory at the University of Tübingen in Germany. “Memories are reactivated and reprocessed.” And just what is it about the sleeping brain that makes it a memory machine? “We don't know how it does any of this,” says Stickgold, “because no one knows how a memory is formed.” But that is not going to stop scientists from trying to find out. Working in humans and animal models, researchers are documenting how the sleeping brain behaves, and trying to link that activity to the vast and complex constellation of information it stores.
A night's sleep has five distinct phases, which the brain cycles through roughly every 90 minutes. In rapid eye movement (REM) sleep, the brain's electrical activity looks much as it does when someone is awake. Researchers assumed that REM was when dreams took place — and that in dreams, perhaps, memories are consolidated, the brain replaying the day's experiences and storing them as enduring recollections.