We were there early last December, and I would have to say that it definitely is cooler at nights than the summer months. How that affected most peoples dress varied. Most women had the same super sexy outfits that they wear in the summer and most men wore long pants in the evening. My wife got body painted with Aquilles and she was not too cold she said, because the lobby area is quite protected from the wind.
HOWEVER, the jacuzzi after the disco was quite cool. The beds really didnt get much use late at night, and the temps in the tub were very welcoming for a change.
FYI,I'm not sure if it was the time of year, or just a fluke, but the bugs were brutal that week for us.
Not so bad though that we aren't trying to get back again this December !!
Steve