Supposedly it's the least productive time of day in the workplace.
Your breakfast needs to slow-releasing energy foods, like muesli or other cereals, with fresh fruit. Then lunch should be a re-fuel, so opt for a combination of quick energy and slow release foods. Avoid any food that is difficult or slow to digest as this makes you feel lethargic.
Take a lunch break and get away from your work area, even if it's for 30 mins. Then take a 15 minute break, or two shorter ones in the afternoon. Pop over and see a work colleague for a chat or just pop to the loo, using the furthest toilet in the building! Anything to refresh your brain, eyes and body.
Finally, if you're drinking tea or coffee during the day, limit yourself to 3 cups (not mugs) per day. 2 preferably. And try to space this out to one in the morning and one in the afternoon. In between, drink plenty of water and herbal/fruit/green teas.
Try this for 2 weeks and I guarantee you won't have that problem anymore.