From an article, The Afternoon Apathy Syndrome, by Simran Bhargava:

Yawn. Your body is feeling leaden, your thoughts are sluggish. Strange glazed circles are forming before your eyes. You literally have to pry your eyelids open to keep awake. All around the office, colleagues are in a similar state of passing away.

Guess what time it is?

ItÂ’s most likely to be three in the afternoon, better known as the post-lunch slump. For the next hour-and-a-half, welcome to the zone of the living dead. The body is present, but the brain is asleep. This afternoon apathy syndrome is very common among office workers and can leave you feeling less alert, less energetic and more apt to make mistakes. Researchers in the UK found that productivity goes down so much during this period, it can have serious financial implications.

Trainers call it the graveyard session and have to fill it with activities to keep participants interested. Seminars typically have the lowest attendance during this time. Meetings held immediately after lunch are full of dead bodies on a caffeine drip. Even daytime accidents are most likely to be caused by lapses in attention which peak mid-afternoon, especially in drivers over age 45.

What is it about 3 pm?

Well, around this time several body cycles clash, sending office workers — not everyone, mind you — into a deep slump. For one, all the bodyÂ’s resources are suddenly diverted into digesting a heavy meal, an act, which according to nutritionists, requires more energy than either running or swimming. Second, a heavy meal causes gastric distension in the stomach leading to a reduced flow of oxygen and energy. Third, by mid-afternoon, a natural sleep-cycle builds up which sends you crashing into a valley. And fourth, at just about this time, the body is also slipping into a boredom cycle: the morning spurt of enthusiasm is over and now, itÂ’s just dull routine work.

The author advises that you can wait it out (it will pass in about 90 minutes), or:

1. Identify your peak energy times (usually mid-to late morning) and use them for your most thought-intense work (planning, writing, editing, creative thinking); in the afternoon, use the time for “busy work” like returning phone calls, sorting papers, reading email.
2. Avoid carbohydrates at lunchtime and eat more protein. Protein releases dopamine and norepinephrine into the brain, which increases alertness, while carbs release serotonin, a natural sedative.
3. Skip the cup of coffee, and take a little nap — no longer than 30 minutes. You don’t have to even sleep; just kick back somewhere quiet, close your eyes, and rest your brain.

Following your body’s rhythms and giving what it needs can greatly improve your productivity and attention, allowing you to reclaim the day.