Melatonin has become the most common sleep supplement given to children, yet its effects on the sleeping brain have remained largely unmeasured—until now. A large study using gold-standard polysomnography found that children taking melatonin spent meaningfully less time in REM sleep, the stage most closely tied to memory, emotional development, and cognitive growth. The finding does not yet confirm harm, but it opens a necessary question: in our eagerness to help children sleep, might we be quietly reshaping the very sleep that shapes them?