You’re not moving furniture or running marathons, yet somehow at the end of the day, you’re exhausted and all you did was sit at a computer. Why are you so tired when you did nothing physically taxing?

There are several theories that attempt to tackle deskside fatigue and why we're drained when the only thing we're moving are our eyeballs across the screen and our fingers around a keyboard. Here are some possible explanations for your desk job exhaustion.

You've run out of mental energy

Even though you're not working your muscles when you're sitting at your desk all day, you are working your brain. Your brain is relatively small compared to the rest of your body, but it's responsible for 20 percent of the oxygen you use.

"Your muscles normally aren’t sucking a lot of oxygen out of you," Steven Feinsilver, the director of sleep medicine at Lenox Hill Hospital in New York City, tells The Cut. "With exercise, they will. But the brain always takes a lot of your energy.” So even if you're just sitting there, your brain is sapping your energy. It's not the same way your body works when you're exerting yourself, but it's still demanding work.

You're not all that motivated

man staring at ceiling while at work Sometimes counting the ceiling tiles is more interesting that your actual work. (Photo: nd3000/Shutterstock)

As we spend the day at the computer or dealing with paperwork, eventually there's a good chance we'll lose focus or get bored with what we're doing. Maybe you click over and check Facebook or you end up texting a friend because that's much more interesting than actually doing your job.

This conflict of doing what you want to do versus what you should be doing can cause tension, which can result in fatigue.

Michael Inzlicht, a University of Toronto psychologist, led a study in which he and his colleagues texted 159 university students throughout the day for a week. They asked them what temptations they were experiencing and how they practiced self control and whether that made them feel tired.

"What was surprising to us was the biggest predictor [of fatigue] was not whether they had exerted self-control," Inzlicht tells Vox. Instead, it was the number of temptations or distractions they felt.

"If you’re typing at work, and if you're anything like me, you got a few browsers open, you got Twitter open. These lead us down these rabbit holes that lead to temptations," he says. These tempting distractions make us less motivated to work, which then can may make us feel tired.

You're stressed out

Work can be hard. You have deadlines and bosses breathing down your neck. Some stress is actually good for you and can be invigorating, but chronic stress can make you feel tired and can affect your overall quality of life, points out Healthline.

So the key is to find ways to deal with your stress. Try deep breathing or go for a walk at lunch. Find a quiet spot to get away from your desk and focus on something else, even for just a few minutes.

Those are short-term solutions, but you also need to figure out what will make your job less stressful and more pleasing over the long haul.

"In the long term, though, you need to find some aspects of work that are actually desirable," writes Ark Markman in Fast Company. "What can you spend some time on that might lead to an exciting outcome? If you can’t find anything, you ought to sit down with your supervisor and find at least one project that will bring you some joy to work on. There should be something that makes you want to work rather than stay in bed every day."

You're sleepy

And finally, one obvious reason you might be tired at your desk is that you're just not getting enough sleep. It's hard to concentrate and focus on work if you're groggy and out of sorts after a way-too-short night in bed. Similarly, sleeping too long can also make you tired during the day.

In general, healthy adults need between seven and nine hours of sleep each night, according to the National Sleep Foundation. We naturally feel tired at 2 a.m. (when most of us are sleeping) and 2 p.m., which accounts for that after-lunch dip. About 29 percent of workers report falling asleep or becoming very sleepy at work, and a lack of sleep costs American companies $63 billion each year in lost productivity.

Because most employers frown on napping on the job, you might want to work on better sleep habits, like going to bed the same time every night, having a bedtime routine and avoiding caffeine in the afternoon and evening.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why are you so tired if you sit at a desk all day?
Here's why you're so exhausted even if you just work at a computer all day.