diff --git a/NEWS.md b/NEWS.md
index ca184d6df..fb827382e 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -11,6 +11,9 @@
   - Improved logging of what guider was used when using NotWorn and optional backup guider, #1156
 
   - Skip night in part 4 csv report if guider was NotWorn, #1156
+  
+- Part 5: Fixed bug in g.part5.wakesleepwindows causing the first SPT window in a recording incorrectly to be  defined from the first timestamp in the recording. For details see: #1192
+
 
 # CHANGES IN GGIR VERSION 3.1-3
 
diff --git a/R/g.part5.wakesleepwindows.R b/R/g.part5.wakesleepwindows.R
index 07862e73c..45f4295dc 100644
--- a/R/g.part5.wakesleepwindows.R
+++ b/R/g.part5.wakesleepwindows.R
@@ -22,11 +22,10 @@ g.part5.wakesleepwindows = function(ts, part4_output, desiredtz, nightsi,
   w0 = w1 = rep(0,length(part4_output$calendar_date))
   # Round seconds to integer number of epoch lengths (needed if cleaningcode = 5).
   round_seconds_to_epochSize = function(x, epochSize) {
-    temp = as.numeric(unlist(strsplit(x,":")))
-    if (length(temp) == 3) {
-      if (temp[3] / epochSize != round(temp[3] / epochSize)) {
-        x = paste0(temp[1],":",temp[2],":",round(temp[3] / epochSize)*epochSize)
-      }
+    if (length(as.numeric(unlist(strsplit(x,":")))) == 3) {
+      xPOSIX = as.POSIXct(x, format = "%H:%M:%S")
+      xPOSIX_rounded = as.POSIXct(round(as.numeric(xPOSIX) / epochSize) * epochSize)
+      x = format(xPOSIX_rounded, format = "%H:%M:%S")
     } else {
       x = ""
     }