diff --git a/mert/TimerTest.cpp b/mert/TimerTest.cpp
index d72b1c312..532e44fc1 100644
--- a/mert/TimerTest.cpp
+++ b/mert/TimerTest.cpp
@@ -11,7 +11,20 @@ using namespace MosesTuning;
 BOOST_AUTO_TEST_CASE(timer_basic_test)
 {
   Timer timer;
-  const int sleep_time_microsec = 40; // ad-hoc microseconds to pass unit tests.
+
+  // Sleep time.  The test will sleep for this number of microseconds, and
+  // expect the elapsed time to be noticeable.
+  // Keep this number low to avoid wasting test time sleeping, but at least as
+  // high as the Boost timer's resolution.  Tests must pass consistently, not
+  // just on lucky runs.
+#if defined(WIN32)
+  // Timer resolution on Windows seems to be a millisecond.  Anything less and
+  // the test fails consistently.
+  const int sleep_time_microsec = 1000;
+#else
+  // Unix-like systems seem to have more fine-grained clocks.
+  const int sleep_time_microsec = 40;
+#endif
 
   timer.start();
   BOOST_REQUIRE(timer.is_running());