Ever get through a long day of work or school and feel both mentally and physically drained even though you spent most of the day sitting at a desk, barely moving a muscle?

The reason lies with your brain’s main fuel source: glucose. Glucose is that deliciously addictive substance that we typically think of when we talk about sugar. The glucose in the food that we eat enters our blood stream and eventually reaches the brain. The brain then uses this glucose to do basically everything – running computations, thinking about anything and everything, telling your arms and legs to move and making sure your body breathes.

Your brain is always working (and always using up glucose). If you use more areas of your brain, for example when trying to figure out the logistics of a complex situation at work or solving a difficult math problem (in addition to breathing and having your heart pump blood), then your brain needs more glucose.

You can think of your brain like your cellphone. When the phone is turned on, but you’re not using it, the phone uses a small amount of battery power. This is your brain performing simple tasks to keep you alive. Once you turn on the phone screen, the battery dies at a faster rate – your brain thinking about something simple.

Try to use more functions of your phone – send texts or make a phone call or play music or do all of these at once – and the battery loses power at an even faster rate. If you keep using your phone without recharging it, your phone will run low on power. Let the battery run low enough, and your phone will most likely go into power saving mode, where the phone doesn’t perform as many tasks as when there is more battery available – your exhausted brain after a long day of work or school.

At this point, you definitely need to take time to recharge your phone (and your brain).

What does this have to do with your body? Well for one, the more glucose your brain uses, the less there is to fuel the rest of your body. Additionally, there’s another substance made by your body, opposite of glucose that plays a role, according to a study in Sports Medicine. As glucose is pulled out of the blood stream and the available amount of glucose decreases, the amount of a substance called adenosine increases. Adenosine prevents the release of another chemical produced by our bodies, called dopamine. Why is dopamine so important? Well, dopamine plays a role in mood – specifically motivation and the pleasure of reward.

How does this all fit together? As your brain works harder, it uses more glucose. As more glucose is used, adenosine levels increase. As adenosine levels increase, your body’s ability to produce dopamine and reward you for your hard thinking decreases. Less fuel is available for the rest of your body, and the satisfaction from additional hard work makes more thinking or doing anything for that matter seem not worth the effort. In other words, your body is tired and telling you to rest.

Note: this doesn’t mean you should recharge your brain by only eating sugar.

Any questions or comments? Send them to me at guido.26@osu.edu.