Photosynthesis has 2 stages.

The light reactions take place in the presence of light, and uses H₂O to make energy molecules ATP and NADPH, releasing O₂.
This takes place during hot daylight hours; plants can close their stomata to prevent water loss.
The Calvin cycle uses the energy molecules from the light reactions to convert CO₂ into sugar.
This requires CO₂ from the atmosphere but not light, so can take place in the cooler night time when plants open the stomata, minimizing the loss of H₂O.