Afternoon Sleepiness occurs generally because of following facts:
1) Low blood glucose level.
2) Taking any sort of mental or physical strain.
3) Not having proper Sleep at night.
4) Skipping the breakfast.
Some of the precautions we can take against it:
1) Have a proper Sleep at night (8 hours).
2) Have a good amount of breakfast.
3) Increase water consumption.
4) Regular exercise which includes some light aerobic exercises.
5) Try Green Tea.
6) Have a less calorific lunch.
7) Avoid alcohols and other stimulants.
8) Try to switch tasks and have a proper amount of break in-between them. Move around, try to listen to some music which will make you feel relax and hence reduce the exhaustion level of your body.