Avoid Feeling Drowsy After Lunch

Let's face it: food can make you sleepy.

How are we supposed to eat a meal in the middle of the day and still finish that second half of the day without dozing off (or at least zoning out)?

Change your diet. Different people will give you different opinions on just how to change your diet to increase energy, but if food makes you tired it's a good bet that food is where the answer is.
Eat low-carb food for lunch. You will feel more awake. Post-lunch drowsiness is caused by your body being flooded with insulin to process that carb-rich lunch you just ate.
Alternatively, don't eat meat, at least not for lunch. Some people are very certain about the connection between meat and being tired.)
Avoid caffeine and processed sugars. (Caffeine gives you an energy boost, followed by a crash that is hard to recover from. Processed sugars are too easily absorbed by the body and tend to pass right through you, burning up quickly and leaving your body starved for energy.)
Eat a nutritional lunch, high in vitamins. Vitamins help make your body healthy, and can help you feel healthy and energised.
Don't overeat. A small meal should not make you drowsy.
Eat many, smaller portions. Snack on healthy food throughout the day instead of eating a lot at once.
Do some light exercise after lunch. Use the stairs instead of the elevator, do a few jumping jacks in the restroom, whatever you can think of. This will help get your blood flowing and will reduce fatigue.
Get plenty of sleep at night. Even if you have a low spot in the afternoon, it won't be as low if you are well rested.
Notice what habits make you sleepy. Write down whether you feel drowsy. Then, write down what you ate, whether you exercised, how well you slept, and any other factors that might have been involved. Then, look for patterns and avoid any habits that cause problems.
Turkey contains tryptophan, which can make you sleepy. However, you would have to eat a huge amount of it to make you very sleepy (which is why so many people are tired after the Thanksgiving meal -- the volume of food.) A turkey sandwich should be OK.
A good alternative to processed sugar is honey. Avoid giving honey to anyone under the age of 2 years.
Though this may not be feasible with your work schedule, you can plan to have a short 15 minute nap after lunch and it will help prevent you from feeling drowsy for the rest of the day.

Consult a doctor before making any major decisions.