Is the meal heavy in fats and carbs at lunch? If so, maybe you can rearrange the meals you eat during the day so that lunch is "lighter".

Could you provide a sample meal of what you might eat? That might help.

I was having the same problem; I went heavier in fats adding more olive oil (2tbsp) and an avocado to my lunch time salads.

Then after an hour or so, have 10-20 grams of dark chocolate and some fruit.

The foods you mentioned are likely high in carbs and protein but low in fats (I assume you're eating lean meats with less than 20% fat). Try raising fat intake during breakfast and lunch, and moving some of your carb intake to dinner.

Add the following: avocados, EVOO, coconut oil, fatty fishes, fish oil, bacon, etc...

Since you can't always pick an optimal meal for lunch, for those days it might be nice to have a cup of coffee or a place to take a quick power nap.

Also, consider that the levels of serotonin, a sleep inducing agent, may rise after a carbohydrate heavy meal:

http://evolutionarypsychiatry.blogspot.com/2011/09/carbs-and-serotonin-connection-after.html, http://www.ncbi.nlm.nih.gov/pubmed/21349213 AND http://ajcn.nutrition.org/content/85/2/426.full

Based on that research, you want to look into supplementing with l-tyrosine, which counteracts the effects of serotonin. Not very paleo, but it works for many.

When I come home from the gym I usually have a banana with yoghurt. Within 30 minutes or so I'm in carb coma but it's only for about 10 minutes. If I don't eat carbs I don't feel the need to nap. Like the others have said above, increase the fats and decrease the carbs. Otherwise, if you are in a position to take a short nap, then go for it. I always feel much better after napping.

Meals high in salt tend to be soporific as well.