It's pretty important KIM,

Plants during lights on take in co2 from the air and give off oxygen.
Ambient levels of co2 in the air is only about 300-350 PPM (parts per million) they can use that amount up rather quickly.
During lights out the plant takes in oxygen and gives off co2 plus moisture, it can raise the humidity a lot.

The easy way to deal with these issues is provide some ventilation.

Time to read up on plant's needs and don't go by your intuition.
OM