There are a group of cells, small but important, in your lateral hypothalamus that contributes to daytime alertness. Loss of these cells is thought to be responsible for the sleep disorder narcolepsy. A group of researchers studied these neurons and the effect of carbs versus protein on their firing rates. They have already shown that carbs inhibit their firing, which could possibly explain 'sugar crashes', where you get sleepy soon after eating sugary foods.
During the current study, the results showed that protein activated the cells, meaning that protein could help keep you awake. Also, protein blocked the effects of sugar on the cells, meaning that you might be less sleepy from eating sugar if you ate it with protein. This makes sense, as diabetics are taught to eat protein with each meal, which could help reduce the negative effects of carbohydrates on insulin levels.
I wonder if people who do low carb, high protein diets are more alert than those on more traditional western diets or high carb diets. That would be an interesting study.