I have observed and used a similar effect in myself. I do my most difficult studying right after eating, then use the long, almost flat plateau afterwards for studying or decision making that requires a high degree of sustained concentration, and try to use the tail for routine chores and R&R. The food doesn't make a lot of difference, the peak seems slightly higher after a high carb snack, but the same effect was present and only a bit weaker when I was eating an almost zero-carb (meat only) diet. Though a sudden change in diet can create unusual effects; for example a really high fiber meal (bean soup) can have unpredictable effects when I haven't been eating a lot of fiber recently.