diff --git a/NEWS.md b/NEWS.md
index 33e2d76bf..2c60c115f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # CHANGES IN GGIR VERSION 3.1-2
 
+- Part 4: Correct calculation of sleep efficiency for `sleepefficiency.metric = 1`, fixed #1157
+
 - Parts 2-5: Give more informative error when folders with expected milestone files are empty. #1144
 
 - Report part 5: fix bug that was introduced on 2024-Feb-19 in the calculation of wear percentage #1148
diff --git a/R/g.part4.R b/R/g.part4.R
index d431deea5..3281eb7c0 100644
--- a/R/g.part4.R
+++ b/R/g.part4.R
@@ -916,9 +916,9 @@ g.part4 = function(datadir = c(), metadatadir = c(), f0 = f0, f1 = f1,
                                                    digits = 7)  #sleeponset - guider_onset
                     # sleep efficiency:
                     if (params_sleep[["sleepefficiency.metric"]] == 1) {
-                      nightsummary[sumi, 26] = round(nightsummary[sumi, 14]/nightsummary[sumi, 9], digits = 5)  #accumulated nocturnal sleep / guider
+                      nightsummary[sumi, 26] = round(nightsummary[sumi, 14]/nightsummary[sumi, 5], digits = 5)  # accumulated sleep in spt / duration of spt
                     } else if (params_sleep[["sleepefficiency.metric"]] == 2) {
-                      nightsummary[sumi, 26] = round(nightsummary[sumi, 14]/(nightsummary[sumi, 5] + nightsummary[sumi, 25]), digits = 5)  #accumulated nocturnal sleep / detected spt + latency
+                      nightsummary[sumi, 26] = round(nightsummary[sumi, 14]/(nightsummary[sumi, 5] + nightsummary[sumi, 25]), digits = 5)  # accumulated sleep in spt / duration of spt + latency
                     }
                   }
                   nightsummary[sumi, 27] = pagei
diff --git a/tests/testthat/test_chainof5parts.R b/tests/testthat/test_chainof5parts.R
index fd4d4fef6..f92399381 100644
--- a/tests/testthat/test_chainof5parts.R
+++ b/tests/testthat/test_chainof5parts.R
@@ -282,7 +282,7 @@ test_that("chainof5parts", {
   load(rn[1])
   expect_true("sleeplatency" %in% colnames(nightsummary))
   expect_true("sleepefficiency" %in% colnames(nightsummary))
-  expect_equal(round(nightsummary$sleepefficiency[1], 3), 0.851)
+  expect_equal(round(nightsummary$sleepefficiency[1], 3), 0.974)
   
   #--------------------------------------------
   # part 4 without sleeplog
diff --git a/tests/testthat/test_recordingEndSleepHour.R b/tests/testthat/test_recordingEndSleepHour.R
index bab2dbe0d..ad9330d31 100644
--- a/tests/testthat/test_recordingEndSleepHour.R
+++ b/tests/testthat/test_recordingEndSleepHour.R
@@ -98,7 +98,7 @@ test_that("recordingEndSleepHour works as expected", {
   expect_equal(sum(p4$guider_inbedStart), 41.42)
   expect_equal(sum(p4$guider_inbedEnd), 57.961)
   expect_equal(sum(p4$number_sib_sleepperiod), 13)
-  expect_equal(sum(p4$sleepefficiency), 0.422)
+  expect_equal(sum(p4$sleepefficiency), 0.428)
   expect_equal(sum(p4$longitudinal_axis), 6)
   
   p5 = read.csv("output_test/results/part5_daysummary_MM_L40M100V400_T5A5.csv")