If you enjoy pondering life's many mysteries, here's a good one: Why do we sleep? You know you need sleep, because that's what your body tells you. But what does sleep actually do? "There are as many theories of sleep's functions as there are sleep researchers," Mehdi Tafti, a geneticist at the University of Lausanne in Switzerland, says in a recent ScienceNews article. The article focuses on recent sleep studies that try to explain the purpose behind sleep, from its effect on metabolism to the role it plays in memory and learning. Whatever your theory is — and there are a lot out there — it's hard to deny the benefits of a good night's sleep.