Here's some neuro trivia for the day (courtesy of Wired):
The reason the orexin system is so important is that it links the needs of the body to the desires of the mind. Several studies have demonstrated that the intake of sugar can decrease the activity of orexin cells, which is probably why we want to nap after a carb heavy lunch. This phenomenon also begins to explain the downward spiral of obesity triggered by our warped modern diet. Because we eat lots of refined sugars, washing down Twinkies with cans of Coke, we continually reduce levels of orexin in the brain, which then reduces levels of physical activity.
I imagine we've all felt this, and I'd bet too that every SLP the world over knows that afternoon sessions are a challenge. So now we know why.