diff --git a/synapse/util/ratelimitutils.py b/synapse/util/ratelimitutils.py

index 0ab63c3d7d..5ac33b2132 100644
--- a/synapse/util/ratelimitutils.py
+++ b/synapse/util/ratelimitutils.py
@@ -13,21 +13,19 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import collections
+import contextlib
+import logging
+
 from twisted.internet import defer
 
 from synapse.api.errors import LimitExceededError
-
-from synapse.util.async import sleep
 from synapse.util.logcontext import (
-    run_in_background, make_deferred_yieldable,
     PreserveLoggingContext,
+    make_deferred_yieldable,
+    run_in_background,
 )
 
-import collections
-import contextlib
-import logging
-
-
 logger = logging.getLogger(__name__)
 
 
@@ -153,7 +151,7 @@ class _PerHostRatelimiter(object):
                 "Ratelimit [%s]: sleeping req",
                 id(request_id),
             )
-            ret_defer = run_in_background(sleep, self.sleep_msec / 1000.0)
+            ret_defer = run_in_background(self.clock.sleep, self.sleep_msec / 1000.0)
 
             self.sleeping_requests.add(request_id)