There's a Reason Your Body Is Tired When Your Brain Is Fried

Ugh, what a day. You were saddled with a million projects, you had to fix a confusing problem with your computer, and you navigated a delicate interpersonal issue with a coworker. You know you said you'd go to the gym after work, but you're wiped out. Even if you get it together to exercise, the workout will probably be lousy. Why, though? Sure, you had a hard day, but it was hard mentally — you were sitting down for most of it. Well, all that stuff they say about the mind-body connection is real. Research shows that mental fatigue really does lead to physical fatigue.

Brain Drain

It's obvious that when your brain is tired, you're not as mentally sharp. But it wasn't until the last decade or so that scientists started realizing that mental fatigue can make your physical performance suffer, too. In 2009, Samuele M. Marcora and his team at Bangor University in Wales published a study in which they had people ride a stationary bike to exhaustion after spending 90 minutes either doing a mentally demanding computer exercise or watching a boring documentary. The participants weren't able to pedal for as long after doing the cognitively demanding computer task as they were after watching the documentary. They also reported higher feelings of physical exertion on the post-computer-task ride.

The team concluded that mental fatigue reduces physical endurance because it increases your perception of effort. More recent research is figuring out exactly why that is. For a study published in June in Sports Medicine, Australian and Belgian researchers examined a hypothesis put forward in 2014 saying that one chemical may be to blame: a metabolite called adenosine.

The idea is that when you do something that's mentally taxing, your brain burns through glucose, the sugary chemical it uses as fuel. As glucose levels drop, levels of adenosine rise, and that blocks the release of the neurotransmitter dopamine. You might know of dopamine as a feel-good brain chemical, and it is, but it also plays a big role in goal-seeking behavior. When dopamine can't do its thing, you end up feeling not only worse overall, but also less motivated to continue the task at hand. The likely reason? Your brain wants you to stop what you're doing and go to sleep so it can recharge.

The new Sports Medicine study found a lot of evidence to back this up. Research shows that adenosine levels rise while you're awake, then drop once you go to sleep. A 2007 study in rats found that adenosine levels rise after intense exercise, and another study a year later showed that if you inject adenosine directly into rats' brains, they were less likely to press a lever to get a delicious food and instead just feasted on less appetizing, but more easily acquired rat chow — the rat equivalent of eating the stale chips in the back of the pantry instead of cooking a tastier meal from scratch.

The Magic Elixir

So there's your explanation: A long day at work or school burns through glucose, which leads to a rise in adenosine, which blocks dopamine and makes every physical task feel harder and less worthwhile. But how do you overcome this phenomenon? It turns out there's a familiar fix: caffeine. "Caffeine is very similar to structure in adenosine and can bind to cell membrane receptors for adenosine, thus blocking their action," the authors of the Sports Medicine study write. Caffeine has been shown to reduce perceived exertion and improve physical endurance, they point out, along with boosting alertness and mood. Even better, combining caffeine with carbs — coffee and a donut, anyone? — has been shown to reduce the perception of mental fatigue, so it's not a stretch to think it might do the same for physical fatigue.

It may help to know that working out when you're mentally depleted might do you good, physically. As Alex Hutchinson, author of "Endure," told us on The Curiosity Podcast, the researcher from that 2009 study is actually experimenting with using mental fatigue training to boost the endurance of professional athletes. "One of the points Marcora makes, the researcher, is that mental training doesn't have to be something you do in the lab in front of a computer. It's a part of all the training you do. So it's like any time you go to the gym and spend an hour at the gym, you're training your body, but you're also training your mind."

The next time work or class leaves your brain fried, reach for a snack and a jolt of caffeine. But most of all, don't beat yourself up too much if you feel physically exhausted. Your legs might not have been working hard, but your brain was, and that's a part of your body, too.

This article first appeared on Curiosity.com.