diff --git a/oscar/Graphs/gYAxis.cpp b/oscar/Graphs/gYAxis.cpp
index d36e7cb7..fd3e97ff 100644
--- a/oscar/Graphs/gYAxis.cpp
+++ b/oscar/Graphs/gYAxis.cpp
@@ -327,6 +327,7 @@ bool gYAxis::mouseDoubleClickEvent(QMouseEvent *event, gGraph *graph)
 
 const QString gYAxisTime::Format(EventDataType v, int dp)
 {
+    // it seems that v is the total duration of the sleep from 12 noon
     int h = int(v) % 24;
     int m = int(v * 60) % 60;
     int s = int(v * 3600) % 60;
@@ -337,9 +338,9 @@ const QString gYAxisTime::Format(EventDataType v, int dp)
 
         h >= 12 ? pm[0] = 'a' : pm[0] = 'p';
         h %= 12;
-
         if (h == 0) { h = 12; }
     } else {
+        h < 12 ? h+=12 : h-=12;
         pm[0] = 0;
     }