"Our uncle goes to sleep very late."
Translation:Ο θείος μας κοιμάται πολύ αργά.
The key thing here seems to be that κοιμάται αργά means the sleep begins at a late hour. How do you translate that into English? It can't be "sleeps late", because in English that means the sleep ends at a late hour. So you need a phrase like "goes to sleep late" - which does indeed feel like the most natural English phrase to be.