Smithsonian Magazine reviews new research on how kids experience a dramatic decline in REM sleep during early childhood.

Their analysis, published today in Science Advances, reveals that the purpose of sleep shifts in children between 2 and 3 years old—from a process of reorganization and learning to a process of repairing the brain from daily stresses and wear.

One reason this study was so successful,” says Van Savage, co-author of the study and biomathematician at the University of California, Los Angeles, “is because it brings together people from different backgrounds, like mathematical physical theory, statistical data analysis, and traditional sleep researchers who look at the data differently than before.”

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