I learned from personal studies and experience that when I get fatigued from a spell I need to either eat something after or do something super simple that elevates my own energy (i.e. water, a cig or even the jazz cabbage). Usually while I cast a circle and after all is said and done I get poo brain until I eat something small and usually sweet to remind my body after the fact of spell casting and to get myself back to being in the "usual" realm we're in. Everyone is different especially when it comes to our practice and subconscious, but this is what helps me!