Does Clojure Ring create a thread for each request?

Ring is the wrong thing to ask this question about: ring is not an http server, but rather an abstraction over http servers. Ring itself does not have a fixed threading model: all it really cares about is that you have a function from request to response.

What really makes this decision is which ring adapter you use. By far the most common is ring-jetty-adapter, which is a jetty http handler that delegates to your function through ring. And jetty does indeed have a single thread for each request, so that you can sleep in one thread without impacting others (but as noted in another answer, threads are not free, so you don't want to do a ton of this regularly).

But there are other ring handlers with different threading models. For example, aleph includes a ring adapter based on netty, which uses java.nio for non-blocking IO in a small, limited threadpool; in that case, sleeping on a "request thread" is very disruptive.