No, we don't have a definitive answer. We are very good at describing what happens when we sleep and what goes wrong when we don't, and so there are a number of theories as to why we sleep. But a definitive answer? There isn't one (yet). Here is the most likely theory according to Wikipedia:
>The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain."[95] This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep, such as memory consolidation or dreaming.
>The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain."[95] This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep, such as memory consolidation or dreaming.
https://en.wikipedia.org/wiki/Sleep