Copy from the Women's World article:

If your tired late morning, the problem could be:
Too much coffee.

Sure, you need your morning cup, but too much coffee produces a sudden spike in your blood sugar that's followed by an even bigger late-morning dip, say Elaine Wilkes, N.C., M.A.,author of Food Choices Made Easy.

If your tired just after lunch, the problem could be:
You've eaten lunch too last.

As you digest, blood is redirected from your brain to your stomach, says Wilkes. When you rush or gulp your food, you don't chew as much-so your digestive system has to work harder, leaving you little energy for anything else. Fight fatigue by: Relaxing! Taking just a few extra minutes to savor your lunch can save you hours of sluggishness. And it not only helps digestion, it can help you shed some pounds too!