Crotonologist:
I am in Delaware so all my crotons winter indoors. Over the years, I have seen quite a bit of variation in which varieties hold up better indoors, and also differences in individual plants of the same variety. Generally speaking though, the more light (and humidity) you can provide, the better. Also, the varieties with the more exotic leaf shapes, seem more delicate and more prone to leaf drop. However, most of my crotons experience some leaf-drop by spring, when they go back outside. Once outside, some will drop more leaves and then, by summer, all look great again.
I currently winter all my crotons in my sunroom, which I heat but not to normal room temperature. When it's cold outside like now (days in the 30s and nights below freezing), the temp in there drops to close to 50'F by morning and then, with sun, warms back up to the 60s. Occasionally, I crank up the heat in there to 70'F+ in winter but the humidity lowers from over 50% to around 35%, as a result, so I don't keep it continuously warm. Most crotons do OK with these conditions, if they have well-established root systems. Smaller plants and more newly rooted plants are more of a gamble in these sub-optimal conditions. For that reason, I do not try to root any crotons after mid-summer.
Although I get sun in the sunroom in winter, the sunroom glass blocks some UV (by design). Crotons can handle this normally but I find my more delicate types appreciate extra light. I bought 2 full-sprectrum lights which I have on a timer for 5-6 hours a day. I put my rarer cultivars under them and they seem to be doing very well (better than in previous years). So I'd highly recommend supplemental lighting for any delicate varieties.
Interestingly, I've found that, by misting my plants with one pint-size spray bottle of water, I can increase the humidity in my 18 X 20 foot sunroom by up to 10% points. It's temporary, but it probably helps.
