sleep

        # Thread::Mutex.sleep

(from ruby core)
### Implementation from Mutex
---
    mutex.sleep(timeout = nil)    -> number or nil

---

Releases the lock and sleeps `timeout` seconds if it is given and
non-nil or forever.  Raises `ThreadError` if `mutex` wasn't locked by
the current thread.

When the thread is next woken up, it will attempt to reacquire the lock.

Note that this method can wakeup without explicit Thread#wakeup call.
For example, receiving signal and so on.

Returns the slept time in seconds if woken up, or `nil` if timed out.