I always get really tired in the afternoon about an hour after lunch, how can i prevent this?

I don't eat processed food and simple carboydrates or foods with artifical colours and preservatives. Nearly everything i eat is natural other than, soya sauce and bread.Today I ate (before i feel tired), porridge, milk, 3 portions of fruit, yoghurt, avocado, bread, philapephia, tomato, rice, salmon, soya sauce, and yoghurt. I do exercise in the morning, either go for a run, go to the gym or go to an exercise cl such as aerobics. I dont like going to sleep in the day because it wastes so much time,and it stops me from sleeping at night so if anyone has any advice please share! :