I don't like the statement "It???s natural for the human body to crave a siesta after lunch" ... this is only true if you do the typical giant american lunch of burger, fries and sugar water, or some equivalent. Most people I work with (and me too until a few years ago) would also admit that was their first meal of the day. It's your body processing this insane and out of place influx of fat and carbs - the crash from the carbs and morning caffeine starts it, and then going into gear to process the fat completely finishes you off.

Since I switched my diet to more natural grains and lean protein, minimizing carbs and sugar (no soda) and eating smaller meals at regular intervals - lunch is usually my third 'meal' of the day - i am actually more energized in the afternoon than i am immediately before lunch.

It definitely helps to get off your keister for a walk after lunch too; especially if you eat at your desk as I do most days.