I've done most of my touring early in the year, including in higher elevation, semi-arid country. Many times the temperature dropped below freezing, especially pre-dawn and into early morning. I've used a tent, thermarest pad, light 32* thermacell bag, and have had no problems even though the odd time it dipped to -5. I attribute it to my wearing wool socks, toque, my cycling leggings, and burning a candle lantern suspended inside the tent. In the desert you'll want to be on your way early, so you should bring warm socks and ski gloves (they don't weigh much but sure are appreciated) It also is important not to use cycling shoes that are a bit too tight. If it is below freezing early in the morning, I sometimes wear a pair of running shoes with the wool socks until the temperature rises.