Different plants get started with photosynthesis at different times when the light comes on.
Some plants use a 'fast start' strategy to out-compete other plants. They can photosynthesize in the early dawn when the light level is still low. It is for these plants that we want the CO2 level to be up there pretty good as soon as the light comes on.
In nature the things that provide CO2 go on 24/7, so the effect overnight is rising CO2. The level is highest in the morning as the sun rises.