Actually, unless it's below 50 degrees, it's healthy for them. Nighttime drops will slow their metabolism down. for some species (montane species) cool nights are crucial. Veileds can benifit, but to a lesser degree.
This is a crude and general description, but it's the basic idea:
think of your chameleon as a lightbulb with a thousand hour life rating. Keep it on constantly, and it'll die at one thousand hours. If you only keep it on during the day, and shut it off at night (by cooling it down) it will last much longer. Again, it's a generalization, but it is applicable.