doc: add overnight test results section to README

gateway/README.md

@@ -96,3 +96,23 @@ estimated from volatile organic compound (VOC) levels rather than directly
 measuring CO2 concentration. Values should be interpreted as indicative trends
 rather than precise measurements, particularly during the first 24 to 48 hours
 of operation while the sensor calibrates.
+
+## Overnight test results
+
+An overnight test was conducted with 2 Thingy:52 nodes placed in two separate
+rooms (windows closed, 7h session, 5 minute interval).
+
+- Room 1: 4 occupants (2 adults, 2 children)
+- Room 2: unoccupied
+
+![Overnight measurements](mesures_nuit.png)
+
+CO2 rose progressively from 400 ppm to a peak of 1071 ppm in the occupied room,
+consistent with human respiration in a confined space. The unoccupied room
+remained stable between 400 and 465 ppm, confirming that variations in Room 1
+are directly linked to human presence.
+
+Note: the CCS811 sensor measures eCO2 estimated from VOC levels, not direct CO2.
+Occasional spikes (e.g. 877 ppm at 02:45, 1071 ppm at 03:05) may be caused by
+factors such as perspiration or movement near the sensor and should be interpreted
+with caution.