A few things to note:
- At high levels of CO2 (usually somewhere above 1000ppm), it can have negative health consequences for humans. I would be cautious about being in there at this point of high CO2.
- Plants have a natural respiration and metabolic cycle, this is why I am not a fan of 24/0 light schedules, plants may not "sleep" like animals, but there are different functions that plants undergo between day and night.
- During the dark period plants are releasing CO2 (whereas in the day time they are primarily consuming it), and taking in O2. The inverse of their daytime functions. Watch as that level sharply drops within the first 1-3 hours of the light cycle.