If you ride for life fitness, and not smaller goals, there is no seasonal decrease. When I was riding a lot, the only problems I had were with seasonal allergies (mostly spring). Other than that, no downtime or seasonal downturn. It got me to the point where I was starting to get irritated with getting dropped by Cat II's. If I'd been able to keep riding as much, maybe I'd be getting irritated with Cat I's by now. Oh well...