[pron]

Thread/sleep will block the entire thread. The thread pool could create more threads, but core.async uses a fixed thread-pool, so, yeah, don't use Thread/sleep in a go block. A timeout channel would be the way to go, or, you can use an alternate core.async implementation, which is a (small) part of the Pulsar project: https://groups.google.com/forum/#!topic/clojure/1xxxTti6Vi0 (I'm the main author)

We will have full API and semantic compatibility when version 0.2 is released next week. Pulsar also has cluster distribution and an Erlang-like actor framework.

Because Pulsar uses instrumentation at the bytecode level, you have more freedom within Go blocks. You wouldn't use Thread/sleep in the Pulsar implementation, either, but Strand/sleep will do the job. It detects whether you're in a go block (implemented as a fiber in Pulsar), in which case it suspends the fiber but doesn't block the thread, or within a normal thread, in which case it will simply call Thread/sleep.