If you've ever thought about laying your head down to rest in the middle of a workday, you're not alone. Called the afternoon slump, the 3 p.m. slump, and afternoon fatigue, this phenomenon is so common that even advertisers use the concept to sell you their products (like Dunkin' and Oscar Mayer).

Related: 4 Reasons Your Body Is Lacking Energy (and How to Get Your Boost Back)

Those commercials may be funny, but continually fighting the urge to doze off with half your day still ahead of you? Not so much. To help you beat the slump, we consulted with a world-renowned general practitioner and lifestyle expert, Dr. Spencer Nadolsky.

Before you all start to doze off, let's dig in.

5 Causes of Afternoon Tiredness (and How to Fix Them)

"Fatigue can come from many things, but the most common are related to lifestyle and lack of sleep," Nadolsky says.

Here are a few common causes of afternoon fatigue to consider.

1. Lack of sleep

This one shouldn't come as a surprise to anyone. By now, we all know that 7-9 hours is the optimal amount of sleep per night. Yet, many people today are quick to give up their time spent asleep for a good old fashioned Netflix binge. What you may not know, though, is that your body actually keeps track of hours of missed sleep. It's called sleep debt, and the more you accrue, the more tired you'll feel midday. So, if you're a chronic under-sleeper, you may need to pay your debt off first before reaping the results of a good night's rest. It's not a sexy answer, but more sleep may nip your afternoon slump in the bud.

How to fix it:

Sleep 7-9 hours a night. Good sleep hygiene starts with having a specific evening routine for the last two hours before bed, according to Nadolsky.

"Many have a bad habit of staying on their phone or watching TV late at night," he says. "If you set a bedtime and shut off electronics an hour or two before bed, that's a good place to start." 

Related: 10 Ways to Stop Sabotaging Your Sleep Hygiene

Other helpful tips: Buy blackout curtains to keep as much light as possible out of your room, invest in a white noise machine to block out disturbing sounds subconsciously, and aim to keep your room at about 67°F (which isn't too hot or too cold).

2. A caffeine crash

Caffeine has a half-life of anywhere between 3-7 hours, but in most people, the effects are only noticeable for 2-4. That's because the human brain is better at sensing changes than absolutes. Four hours after consuming caffeine, what the brain senses is not that it has more energy than it would without caffeine, but that it has less energy than it did an hour ago. The more caffeine you consume, the more pronounced this effect is, and the greater your crash will be.

How to fix it:

Limit caffeine, but you don't have to entirely eliminate it. Caffeine addiction can occur with doses as low as 100 milligrams per day, so you should aim to drink two or fewer cups of coffee, tea, soda, and anything else that has caffeine in it. Try to stop consuming caffeine no later than noon, so that it won't interfere with your sleep.

3. A blood-sugar crash

Eating food will raise your blood sugar for a while, then cause it to fall later. Certain foods, like those that contain refined carbs and sugar, will create a significant and rapid spike in blood sugar, followed by a quick crash. Other foods, such as those low on the glycemic index (which is used to measure the effect of food on blood sugar), will provide you with more sustainable energy.

How to fix it:

Eat healthy, slow-digesting foods — that is, foods that are low on the glycemic index. Stick with lean meats, fresh fruits (not fruit juice), vegetables, and fibrous carbs like beans or oatmeal — not bread and pasta. This is important not only with lunch, but also with breakfast, since a healthy breakfast will bolster your energy levels throughout the day.

4. Stress

Studies show that work-related stress (and stress in general) can prevent you from relaxing, so you never really feel fully rested.

How to fix it:

Reduce stress at the source. Stress not only comes from work, but from money and relationships, too. Nadolsky suggests that you start by reducing the number of commitments you have.

"Sometimes it comes down to taking a task or two off your plate, whether it's work or extracurricular activities," he says. "Also, learning to just say 'no' to something someone asks you to do."

Related: 6 Simple Ways to Overcome Stress and Anxiety

5. Medical issues

"Hypothyroidism (aka an underactive thyroid), anemia, and depression are common causes of low energy that should be screened for," Nadolsky says.

Thyroid hormone and blood-iron levels should be part of standard medical bloodwork, and are worth testing every year or two.

How to fix it:

If afternoon fatigue is a severe and persistent problem for you, then you should seek professional help. Nadolsky says the DIY route won't work for everyone.

"Work with a doctor who can get to the root of the problem instead of just guessing," he explains.