Listening in on the electrical currents of teenagers' brains during sleep, scientists have begun to hear the sound of growing maturity. It happens most intensively between the ages of 12 and 16 1/2: After years of frenzied fluctuation, the brain's electrical output during the deepest phase of sleep becomes practically steady.
That reduced fluctuation in electroencephalogram signals during delta-phase sleep appears to coincide with what neuroscientists have described as major architectural changes in the brain that pave the way for cognitive maturity.
While babies, toddlers and young children are taking in and making sense of the world, their brain cells are wiring themselves together willy-nilly, creating super-dense networks of interwoven neurons. But as we reach and progress through adolescence, neuroscientists have observed, a period of intensive "synaptic pruning" occurs in which those networks are thinned and the strongest and most evolutionarily useful remain.