diff --git a/src/asleep/sleep_windows.py b/src/asleep/sleep_windows.py
index 7d4e281..d570d0b 100644
--- a/src/asleep/sleep_windows.py
+++ b/src/asleep/sleep_windows.py
@@ -272,7 +272,10 @@ def get_day_intervals(start_date, end_date, date_format):
     my_day_end = pd.to_datetime(day_end_str, format=date_format)
     my_day_start = start_date.replace(
         hour=12, minute=0, second=0, microsecond=0)
-    my_day_start = my_day_start - timedelta(hours=24)
+
+    # if the start date is before 12:00 noon, then the interval start is 24 hours prior
+    if start_date.hour < 12:
+        my_day_start = my_day_start - timedelta(hours=24)
 
     while my_day_end < end_date:
         my_day_end = my_day_start + timedelta(hours=23, minutes=59, seconds=59)