The automatic night light is meant to turn on automatically at night as soon as you hop on your bike. To turn on this feature, simply press the switch at the bottom.
The light will turn on when SmartHalo is connected to your phone AND there is movement on the device. It will not turn on during the day, only at night.
And it turns off by itself when you're done riding. It will turn off automatically either if you disconnect OR if the device doesn't move for a while. (Or if you bike all night and you're still up when the sun rises...)
You can always turn on the light manually by making two short taps on the surface of SmartHalo (except when SmartHalo is charging, where tapping will show you progress).