Generally speaking your body needs to get about a gram of protein per pound of muscle a day just to maintain it, and more to build with, and if you're not exceeding your calorie requirement for the day it's not going to happen. You should be eating ~20 minutes after working out, high on protein, like a protein shake if you're not at home. But a lot of people get hungry because they're not drinking enough water, or rather, if they were drinking enough water they'd not be hungry. It's really best to consume a couple of liters of water/gatoraid during the period prior to working out through about a half hour afterwards, and maybe some coffee, both of which will tend to suppress appetite. Especially since you're doing cardio-intensive if you're doing circuit training, rather than lots of weight lifting. I'd usually kill a cup of coffee and a liter of water prior to cardio in the mornings, heavy on the sugar or honey in the coffee, then eat a monster breakfast afterwards. But while a lot of people overdo the whole hydration thing, most people get minimal benefit from exercise simply because they're not getting enough water for their system to process the fat and protein.