You can to manually or automatically set the display brightness of your watch. To do so, perform the following steps: 

1. Open the Withings App.
2. Tap your Devices in the upper right.
3. Find your device and scroll down to "Display Options".
4. Toggle Auto-brightness.
   When on, the display brightness will vary automatically depending on the outside light. 
5. You can also move the Brightness slider to set it manually. 

Note: When a workout or sports session is launched on the Steel HR / Steel HR Sport, the display of the watch will remain on, at full brightness, until workout mode is ended. This is normal behavior and does not indicate an issue with the watch.