Your thermostat's display brightness is at its maximum level when you're using it. When your thermostat is in sleep mode, the display brightness is automatically adjusted by default based on ambient light. You can change the display brightness of your thermostat in the Home + Control app: go to Settings, select "Manage my home", select the room where the thermostat is installed, select the thermostat and select "Advanced", you can then turn off the display brightness permanently or set a custom level.