Sleep in babies seems to mainly help develop new brain connections, but at the age of around two-and-a-half there is an abrupt shift to brain repair
Source:
https://www.newscientist.com/article/2254804-the-purpose-of-sleep-appears-to-change-when-we-are-toddlers/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home