diff --git a/server/internal/services/metricsweep.go b/server/internal/services/metricsweep.go
index 5e44be9..90df9ee 100644
--- a/server/internal/services/metricsweep.go
+++ b/server/internal/services/metricsweep.go
@@ -23,6 +23,15 @@ func EnsureMonitorServerStateIndexes() error {
 	if err != nil {
 		log.Printf("warning: monitor_server_states indexes: %v", err)
 	}
+	// Every heartbeat ping looks its monitor up by token hash. Sparse because
+	// only heartbeat monitors carry one.
+	_, err = db.Col("monitors").Indexes().CreateOne(context.Background(), mongo.IndexModel{
+		Keys:    bson.D{{Key: "heartbeat_token_hash", Value: 1}},
+		Options: options.Index().SetSparse(true),
+	})
+	if err != nil {
+		log.Printf("warning: monitors heartbeat_token_hash index: %v", err)
+	}
 	return nil
 }