Now that we are having triple digit temperatures, I'm back to doing my gardening in the dark. It seems most gardeners in the desert prefer early morning for garden chores, but not me. I don't like getting up at dawn for any reason, even for my cacti, which get about as much attention as others lavish on their pets.
Evening watering seems to agree with my potted cacti and succulents as much as it does with me. There's less evaporation, no heat stress, and the superheated pots have had time to cool a bit before adding water. Landscape cacti do not need any supplemental water yet, but the potted specimens are averaging twice a week now with the high temperatures and low humidity.
Cacti and succulents do all their work at night anyway. Because they are Crassulacean Acid Metabolism (CAM) plants, they wait until nightfall to open their stomata to allow for gas and water vapor exchange. Other plants do just the opposite, opening their stomata during the day. I just barely understand the complicated process. I do know that CAM is the reason cacti and succulents are slow growers and are adapted to extreme heat and drought. Makes me wish I had a CAM process!
I'm not positive of the science involved, but my potted cacti seem to thrive and grow, even in the hottest part of summer, just when they are expected to stop. I'm convinced it's because I water them at night. Actually, the only science involved here is psychology. It's my subconscious justification for sleeping late.