I've been out on my bike when it was 23 degrees Fahrenheit once, and lots of other times in the upper 20,s, and I have noticed that it makes me a lot more fatigued. Even on one of our short rides which are 15-20 miles, I will get bleary eyed and sleepy for the rest of the day. I think maybe it burns more calories because of the cold, or it just makes me feel cold and stiff.