It has to do with air density and the air/fuel mixture. When the air is colder (night time) it’s dryer and more dense, and when it’s warmer (day time) it is less dense and can hold more moisture.
So during the day your mixture will become slightly richer, or vice-a-versa, at night the mixture will get leaner; and everybody knows "leaner is meaner".
You could counter act this by tuning for the time of day you ride most, lets say day time, so if your running good at night now, try down jetting one size for day time riding, and at night cover 1/2 of your air filter to richen the mix to compensate for the cooler air. Or if you ride at night, tune with