In very early life, sleep helps build the brain’s infrastructure, but it then takes on an entirely new decluttering role.

Prolonged sleep deprivation can lead to severe health problems in humans and other animals. But why is sleep so vital to our health? A UCLA-led team of scientists has answered this question and shown for the first time that a dramatic change in the purpose of sleep occurs at the age of about 2-and-a-half.

Before that age, the brain grows very rapidly. During REM sleep, when vivid dreams occur, the young brain is busy building and strengthening synapses — the structures that connect neurons to one another and allow them to communicate.