Try changing your Blueprint to this:
You’ll need to create the float variable “Night Speed.” It isn’t truly the speed relative to the Day Speed, but I recommend you make the Night Speed 5% the size of the Day Speed.
To code works by detecting the “Nightness”, then adjusting the speed based on that. Also, you’ll really need to make the Night Speed really low if the sun doesn’t get very high in the sky.
Hope this helps!