You're working hard, so you push the thought away, but soon it comes back, louder and more urgent.

-You start thinking about the bar in your backpack, how delicious it would taste, and pretty soon it's all you can think about as your brain chants louder and louder. -Chocolate.

Chocolate!

Until you cave in and give it what it wants.

Sound familiar? You're not alone.

Maybe it was bacon, or pizza, or a chai latte, but nearly everyone has felt food cravings at some point.

But why?

And is this unshakeable desire for a chocolate bar my body's way of telling me I need more magnesium?

Well, before we talk about cravings, let's look at hunger.

Actual, physiological hunger is how your body tells your brain it needs a resupply of calories.

It isn't about specific flavors, it's about survival.

When your stomach is empty and your blood sugar levels are dropping, your body starts secreting ghrelin, otherwise known as "the hunger hormone.”

The ghrelin tells the hypothalamus in your brain that you need to eat, and triggers a chain reaction that revs up your appetite and gets your digestive system ready to receive food.

And then once you've gobbled down enough food to stretch your stomach a bit, the ghrelin tap turns off, and you feel satiated - for a while.

But, if you've ever been tempted by the dessert menu even after a big meal, you know that food cravings are different from straight-up hunger.

In some cases, a food craving can be a sign that you need more of a specific nutrient.

If you're very low on salt, for example, you might crave potato chips or pretzels.

But if you need something more specific - say, magnesium - there's no evidence that you'll start craving chocolate, even though chocolate has magnesium in it.

Often, cravings are more about a psychological hunger than a physiological one, and they can be wrapped up in emotions like stress and anxiety.

In the past, we've talked about how humans tend to be drawn to fatty, high-carb foods, in part because they're loaded with the energy we need to think and move and do stuff.

Sure, that's a physiological need, but the pull toward junk food is often a psychological one as well.

Eating a butter-frosted cupcake or bag of salty French fries releases an opioid typhoon that lights up the brain's pleasure center and makes us feel awesome, at least for a little while.

Celery just doesn't have the same effect.

Cravings are also tied to your brain's memory center, which explains why you might also crave a food that isn't full of fat or sugar.

Your brain could be tying that food to a happy memory, or a feeling of reward.

And thinking about a memory associated with a food can make you crave that food.

Another psychological cause of cravings can be a boring or restricted diet.

In studies where the subjects had to drink only meal replacement shakes, they ended up with a lot more cravings than usual, especially for solid foods.

And this diet provided all the calories and nutrition that the subjects' bodies technically needed, so they probably didn't get those extra cravings because they were missing out on some nutrient.

Pregnancy hormones, on the other hand, probably don't cause cravings.

Pregnant women are often portrayed as having serious urges for things like pickles and ice cream, but there isn't much evidence for a connection between those hormones and cravings.

Instead, these cravings probably have more to do with mood.

So, if you're really jonesing for that chocolate, it's probably just because your brain remembers that it feels good to eat it.

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