at dark the plants will droop, its rest and recuperate time. they work hard when the lights are on. when my lights and co2 gen shut off my co2 in the room is at 1100ppm, when lights came on tonight it was 1023ppm. if lights were on and co2 gen was turned off it would go below 100ppm in a few hours. ive walked in when propane ran out during the light cycle and was reading 0. this alone says the plants are not processing anything at rest time.