The afternoon slump would be OK if you could just lie down for a little nap. But most of us have to earn a living, and management would likely frown on anyone who went home from 2 to 4 p.m. for a siesta.
Unless (a) you’re somewhere in Europe – where this is perfectly acceptable or (b) you have the total freedom to create your own schedule every day, then an afternoon nap is out of the question.
So how can you cope with an urge to sleep after lunch?
This article suggests ways on how you can beat the afternoon slump.
The urge to sleep in the afternoon is natural.
According to Ms Lona Sandon, an assistant professor of nutrition at the University of Texas Southwestern Medical Center, “there seems to be something natural about this lull. Some cultures have the siesta, and people find that they’re more productive and better able to concentrate if they take time off after lunch and come back later.” 
She adds that there are two processes at work in the mid-afternoon sleep slump. She said the body’s natural rhythms plus eating habits can combine in a “double whammy leading to a massive energy crash.”
Let’s explore other reasons why you may be feeling drowsy after lunch:
Some people eat too many simple carbohydrates like white bread, white rice, pasta made from white flour, and chips. Eating simple carbs can cause the body to produce a big spike in blood sugar. The elevated sugar in the blood can give energy at first, but it is quickly metabolized and the body crashes.
Tip: Why not swap simple carbs for complex carbs like whole-grain breads, crackers, pasta and brown rice?
Trying to beat that deadline or doing several phone calls in a day could make you easily forget to move and walk around. The human body reacts to stillness with an urge to sleep.
Tip: Try stretching or walking around a while every hour or so at least. Or maybe it’s time to have that standing desk set up?
Even minor dehydration can cause a person to be moody or to feel like sleeping. A loss of just 1.5 percent of the body’s water can bring on drowsiness.
Tip: Keeping a glass of water at your desk and sipping some throughout the day. Better yet, bring that tall jug and keep it next to you while working so you’ll be prompted to drink water and finish it (you’ll end up drinking more water compared to having just a glass).
Your core body temperature drops between 2 p.m. and 4 p.m. daily in a natural cycle called the circadian rhythm. The body reacts to the lowered body temperature with a release of melatonin, a sleep hormone.
Tip: Try listening to upbeat music, going for a brief invigorating walk or going outside to be in the sunshine for a little while.
MayoClinic.org suggests other causes of drowsiness in the afternoon.  Some of them include:
Or it may just be because you’ve had too much for lunch or your workplace environment is really eerily quiet!
In an interview with WebMD, Professor Sandon gives the following recommendations on how to fight the afternoon slump... through a proper diet:
Which brings us to caffeine. Reader’s Digest, in its article “18 tricks to beat the afternoon slump at work,” recommends a cup of caffeinated tea.  But what about a stronger jolt of the world’s most popular consciousness-altering drug (caffeine)? Why not a strong cup of coffee or a couple of caffeinated Viter Energy Mints when the Sandman sprinkles sleep dust on your workday?
Much research has been done on coffee and caffeine and how they stimulate alertness and feelings of well-being. As long as you take caffeine in moderate amounts, it is safe and even beneficial. Also, many articles say taking caffeine too late in the day can interfere with sleepiness at bedtime.
Here's the deal. As long as you don’t have too much caffeine in a day and don’t take it too late, there’s no reason not to have a cup of joe in the early afternoon. You might overcome the afternoon doldrums while enjoying the world’s favorite mood-altering substance.
Or if push comes to shove, maybe you can use 20 minutes of your lunch break for a quick “caffeine nap,” which many people attest to.
If you’d like to know how it works, as well as how to use caffeine to your advantage (vs. an afternoon slump), check out this article: How does caffeine keep you awake?
Is there a big difference between synthetic and natural caffeine? Which gives a stronger jolt? Does it even matter?
Natural caffeine in coffee, tea, and chocolate is much less common than the synthetic caffeine found in so many other products.
Caffeine is found in plant species such as the more popular ones like Coffea arabica and Coffea robusta, as well as tea leaves, kola nuts, cacao beans, Yerba mate and guarana berries.
Not only does naturally-occurring caffeine from said plants keep your cognitive functions at their peak, but it also contains antioxidants that help you fight illnesses like cancer and Alzheimer’s.