RSpec: stubbing Kernel::sleep?

If you are calling sleep within the context of an object, you should stub it on the object, like so:

class Foo
  def self.some_method
    sleep 5
  end
end

it "should call sleep" do
  Foo.stub!(:sleep)
  Foo.should_receive(:sleep).with(5)
  Foo.some_method
end

The key is, to stub sleep on whatever “self” is in the context where sleep is called.