You think your brain runs on sugar. Half right.

Sugar is the fuel. Fuel does nothing until something lights it.

Your brain is about 2 percent of your body weight. It burns roughly 20 percent of your energy at rest, most of it from glucose.

That is a small engine pulling a lot of fuel. And every bit of that glucose has to pass through one gate before it becomes energy you can think with.

The gate is an enzyme called pyruvate dehydrogenase. It cannot open without a vitamin most men have never given a second thought.

Here's something most coffee lovers never think about.

Caffeine doesn't really give you energy.

It works by blocking a natural compound in your brain called adenosine.

From the moment you wake up, adenosine slowly builds up in your brain. The longer you stay awake, the more of it there is, and the sleepier you feel. Scientists call this "sleep pressure."

By bedtime, that pressure is supposed to be strong enough to carry you into deep sleep and keep you there.

Caffeine simply hides the signal for a few hours.

But plenty of everyday things can weaken that pressure or override it. A late-afternoon latte. A long nap on the couch. A stressful day that keeps your mind buzzing long after the lights go out.

And you feel it: tossing and turning, waking up at odd hours, dragging yourself through the next morning.

The encouraging part? Adenosine is part of your body's natural rhythm, which means you can work with it instead of against it.

It's not a sleeping pill, and you can try it tonight.

If you've been waiting for the night when you finally sleep straight through, this is a good place to start:

>>> Learn How to Work With Your Brain's Natural Sleep Signal

Thiamine, Also Called Vitamin B1

Your cells turn it into the active form that switches on the enzymes pulling energy out of glucose.

Plenty of fuel, no match. That is what running low on thiamine looks like.

Let me tell you how we figured that out. Back in the 1880s, the Japanese navy was losing sailors to beriberi: weakness, numb legs, hearts giving out.

A navy surgeon named Takaki Kanehiro noticed the officers rarely got it. The enlisted men lived mostly on polished white rice.

He changed the rations and added barley, meat, and vegetables. The disease nearly disappeared from the fleet.

He thought it was protein. It was the thiamine, which sits in the outer layer of the rice grain that polishing strips away.

They milled the fuel and threw away the match. Plenty of us are still doing a version of it with lunch.

You are not a sailor on a rice ration. But the same chain runs in you, and three things pull on it.

First, the more carbs you eat, the more thiamine you burn processing them. A bread basket and a pasta lunch are not just fuel; they are a withdrawal.

Second, alcohol. It interferes with how your gut absorbs thiamine and how your cells use it.

Third, some diuretics, the water pills prescribed for blood pressure or fluid, increase how much thiamine you flush out. Do not touch the prescription; just know it is part of the picture and bring it up with your doctor.

Now stack them. Rolls with breakfast, a sandwich at noon, two drinks last night, a water pill with the morning coffee.

Demand up. Supply down.

And your body does not keep much in reserve. Total stores sit around 25 to 30 milligrams, enough for a few weeks, and it is water-soluble, so it does not hang around.

Your brain does not run out of sugar. It runs short of the ability to use it.

Research suggests early low thiamine shows up as vague stuff. Tired, irritable, foggy, short on focus.

Look at that list again. It is the exact one most guys file under “getting older.”

So that is how you can eat a full lunch and still sit in a 3pm meeting with a brain trying to light wet wood.

And that meeting is where the money gets made or lost. The deal, the hire, the call you have to get right the first time.

Food covers a good share of it. Pork, beans, lentils, sunflower seeds, whole grains.

But if your week stacks the demand side, a quality B-complex is worth a look. So is benfotiamine, a fat-soluble form of B1 your body absorbs better than standard thiamine.

Look at yesterday honestly. Count the refined carbs, count the drinks, and check whether a water pill sits in your cabinet.

If two of those three show up, B1 belongs in your plan. Bring it up at your next checkup too.

Your income runs on that brain. Every decision you get paid for passes through a gate held open by a vitamin that costs pennies a day.

Hell of a place to run short.