For optimal energy levels, lunch should be eaten at midday, not midafternoon. If you wait too long, not only are you likely to overeat, but you will also feel extremely wiped out. A late lunch typically leads to a late dinner, which will interfere with your ability to sleep. A light, high energy midafternoon snack is fine. If you need more energy for the afternoon, make your power lunch more powerful by concentrating on protein, keeping your starch at a minimum, and keeping the whole meal a bit smaller. Eat very slowly for optimal absorption, and do not drink alcohol.
(Source: From Tired to Terrific in 10 Days: Natural Energy, by Erika Schwartz, M.D., and Carol Colman.)