With Night Temperature slider you select the Color Temperature of the screen during the Night.
Iris detects if it’s day or night based on the clock and location. If you don’t see change when you move this slider it’s probably day.
The value is measured in K (Kelvins) and this is the standard unit for Color Temperature.
The lower the value the less blue light is emitted from the screen.
The bigger the value the more light is emitted from the screen.
Basically high values are bad for the eyes and sleep and low values are good.