i could see the plants close proximity making you sleepy. think about it. a few big healthy plants in their peak turning what co2 there is into nice pure o2, maybe even dropping the rooms total co2 ppm, don't know if they use more than you put off.
anyway o2 makes you sleepy and calms you. one reason they recommend houseplants to urban dwellers and people with depression/anxiety.
also airplanes give you a nice boost of o2 right before you crash and burn, go a little more peacefully