Back in August during our trip to Colorado, it got down to 38°F one morning ride. When the body is used to Summer temps, this comes as a shock to the system. It's usually 2-3 ~40° rides before I even want to think about getting outdoors once on a bike when it eventually gets below freezing. Then a couple of freeze-level rides before the body realizes it can handle a little bit more (less rather). Got in a good hour+ before the work week. 23° felt like a sweet spot. Two performance Tshirts, arm warmers, then a long sleeve wool jersey and I was fine and toasty. Toes are my worst problem, but they are expendable. Ride yer bike!