During the dark cycle the plants do not photosynthesize so they have no need for CO2. They continue their other life processes, however. The cells continue to function and sugars are used to fuel their energy needs. As a result they respire, absorbing oxygen and releasing CO2.
When you open the closet in the morning you are getting a whiff of air enriched with CO2 over the course of the dark period. The CO2-laden air mixes with the room air and the CO2 concentrate dissolves.
When you open the door before the light period begins you let out the elevated concentration of CO. Instead, let the door stay closed for at least a half hour after the lights go on so the plants can take advantage of the CO2 enriched air. Photosynthesis and growth will accelerate until the gas is used up. Open the door and you will sense very fresh oxygen laden air gushing from the closet. The room air, containing normal amounts of CO2, will fill the garden space.
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