I allow mine to drop down to 50 nights routinely for many years with no problem. This fall and last fall I allowed temps to drop even lower for my veileds- nearly 40 for adults, 45 for babies. Still no problems. This fall my veileds experienced those temperatures for a few weeks. Not nightly- some nights low 60s, many low 50s and some 40s, with only a few lower 40s. Day temps I personally don't worry about too much. Because after a few weeks the temps drop below this at night and I move them indoors anyway. I have moved them early one or two years because of rainy along with cool weather for several days with temps that remain in the 50s and 60s- but if sunshine is available they are good at doing their thing even if day temps remain pretty low. And if it is an occasional day or two or three at cooler temps without sunshine, they are fine.
Parts of their range in the wild experience occasional night frosts- though they are reported to dig into the earth or shelter among large rocks and presumably are able to otherwise locate microclimates to avoid actually freezing temperatures.
Personally- I'm uncomfortable going below the 40s for them. I haven't done it and have no plans to do so in spite of conditions in nature.
I also am a little uncomfortable recommending temps below 50 to someone who really doesn't know what they are doing. If you have poor husbandry in some way and a weak animal, even though it may not appear weak to a beginner, it would probably be pushing it to drop below 50. But having used 50 as my low for many years prior, I feel very comfortable recommending that as a safe cut-off.
Keep in mind its a low temp cham, so once the temps start creeping into the upper 80's make sure his has lots of shady spots.
I wouldn't agree with the first part of that sentence, but certainly the last part. Summer temps in nature can climb above 100 in it's native range. But availability of cooler microclimates in the terrarium are certainly important for the well being of this lizard.
