I've also been thinking that a short fast, say 15h ended with a workout is equivalent to longer fast, say 20h. Meanwhile long fasts, say 20h ended with a workout can be too much and are something to perhaps be avoided. Instead, a pre-workout meal would be best with amounts of food according to your goals.
I say this under the notion that with the drop in body temp, mental fogginess and so forth could bring a slight drop in performance, sub-par sessions and problems with "getting into it". Personally I've found it hard as hell to warm-up in a cold gym when you're already freezing cold from a long fast.
Interesting observation. I'd agree - good energy to workout at 15-18 hrs for me; wait longer and it gets hard. I haven't tried a long fast/pre-workout meal yet, but I'll give it a go soon.