Why do babies sleep so long? If it is so that there body can get time to develop, why isn’t that time spent in the womb instead?
Human babies are born underdeveloped; if they stayed until fully developed (like for example young horses), their heads would be too big for natural birth. So being underdeveloped, they spend most of the time sleeping since they can’t walk, can’t run, their senses are still developing etc etc etc
You may have noticed that the womb is actually inside a human being. As such, there are enormous costs and risks to keeping a fetus inside a woman’s body for a prolonged period of time. Research has shown that the burdens placed on a woman from growing a fetus are enormous, and risks go up dramatically as the fetus gets larger. The conventional wisdom is that babies are born when they’re born because there’s a tradeoff between size of the baby’s head and width of the hips. But more recent research has shown that it would be too much of a burden on a pregnant woman’s body to stay pregnant for much longer. Just too high of a cost of resources.
Cell recovery and growth accelerates when you sleep, that’s why teens have to get a lot of sleep too.