For those of us that work in cubicle farms.
#1 There are three main reasons for sleepiness post-lunch:
1.The food you have eaten is diverting your blood for the digestion process.
2. You haven’t had enough sleep.
3. You are unfit, or you have an illness.
2. Eat a good breakfast.
3. Watch what you eat.
4. Eat less.
5. Skip the wine or beer with lunch.
6. Curb your caffeine intake after lunch.
8. Sleep adequately.
9. See your doctor.
10. Notice what habits make you sleepy in a “food/mood diary”.
See the more in depth descriptions at WikiHow