diff --git a/cmd/syncthing/main.go b/cmd/syncthing/main.go
index 058df843..724c259b 100644
--- a/cmd/syncthing/main.go
+++ b/cmd/syncthing/main.go
@@ -269,7 +269,7 @@ func defaultRuntimeOptions() RuntimeOptions {
 	}
 
 	if os.Getenv("STTRACE") != "" {
-		options.logFlags = log.Ltime | log.Ldate | log.Lmicroseconds | log.Lshortfile
+		options.logFlags = logger.DebugFlags
 	}
 
 	if runtime.GOOS != "windows" {
diff --git a/lib/logger/logger.go b/lib/logger/logger.go
index 5d927fe4..d09b8662 100644
--- a/lib/logger/logger.go
+++ b/lib/logger/logger.go
@@ -28,6 +28,8 @@ const (
 	NumLevels
 )
 
+const DebugFlags = log.Ltime | log.Ldate | log.Lmicroseconds | log.Lshortfile
+
 // A MessageHandler is called with the log level and message text.
 type MessageHandler func(l LogLevel, msg string)
 
@@ -215,6 +217,7 @@ func (l *logger) SetDebug(facility string, enabled bool) {
 	l.mut.Lock()
 	l.debug[facility] = enabled
 	l.mut.Unlock()
+	l.SetFlags(DebugFlags)
 }
 
 // FacilityDebugging returns the set of facilities that have debugging