Behavior of plant guard cells: open or closed stomata
I found a paper and this is the link https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3258058/  it says that Stomatal opening is induced by low CO2 concentrations, high light intensity and high humidity. By contrast, closing is promoted by high CO2 concentrations, darkness, drought and the plant hormone ABA.
Another paper, Open or Close the Gate – Stomata Action Under the Control of Phytohormones in Drought Stress Conditions https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3652521/ 
During the night, elevated levels of CO2 in the leaves were observed due to respiration. It has been proved that CO2 has a positive effect on the stomatal closure process