diff --git a/tests/visualization_tests/test_timeline.py b/tests/visualization_tests/test_timeline.py
index c7f124a2c3..443ee86d9d 100644
--- a/tests/visualization_tests/test_timeline.py
+++ b/tests/visualization_tests/test_timeline.py
@@ -158,6 +158,9 @@ def _objective_with_sleep(trial: optuna.Trial) -> float:
         # Spin waiting is used here because high accuracy is necessary even in weak VM.
         # Please check the motivation of the bugfix in https://github.com/optuna/optuna/pull/5549/
         while datetime.datetime.now() - sleep_start_datetime < datetime.timedelta(seconds=0.1):
+            # `sleep(0.1)` is only guaranteed to rest for more than 0.1 second; the actual time
+            # depends on the OS. spin waiting is used here to rest for 0.1 second as precisely as
+            # possible without voluntarily releasing the context.
             pass
         assert datetime.datetime.now() - sleep_start_datetime < datetime.timedelta(seconds=0.19)
         trial.suggest_float("x", -1.0, 1.0)