I would have to think less energy, because we're already so inefficient. For me, a 40-mile ride @ 90ºF I might intake +50oz of water, while the same ride @ 50º might be 8oz of water. I'm producing the same amount of waste heat both days, but on the colder day I can actually use that heat-- I don't need to sweat buckets to accommodate it. At least to a point. Below 30ºF I always feel like output is way down-- my body just doesn't like it that cold. Once the hands and feet get chilled through, I feel like I'm just trudging along.