SmartNight check some conditions like ambient sound and phone movement in order detect when the phone need to be set in night mode.
Checks are made at a user specify frequency. In order to limit false positive set time zone when night mode need to be detected. This also limit battery used.
Depends of device, sensor can be disable when screen is off. Then only ambient sound is used.
For more accuracy and less false positive check, complete this Locale condition by others plug-in like: location, dock detection, etc...