Quote Originally Posted by beccaB View Post
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.
Good for all you riding in the cold!

There is huge debate about why riding in extreme cold makes you feel more tired, but everybody agrees -- riding in very cold temperatures is orders of magnitude more difficult than riding the same route in moderate or warm temperatures. Here's an interesting discussion about slower winter riding.

Happily, since I moved to the Pacific Northwest, I've not had to worry about anything below about 35°F all winter. Today it was 37°F and raining with a moderate south wind (I ride south in the morning, naturally) -- not exactly delightful, but perfectly fine with the right clothes.