Those who spend most days sitting in front of a computer screen probably wonder why they often feel exhausted at the end of the despite doing little (if any) physical work. So why is your desk job making you so tired?

Turns out, fatigue has more to do with motivation than physical exertion, according to two recent studies. The first tracked 100 British nurses through their workday and evaluated how tired they felt afterward. Researchers discovered that physically demanding jobs didn't necessarily make people feel fatigued.

"In some people, physical activity is fatiguing," Derek Johnston - the Aberdeen University psychologist who led the study - explained to Vox. "But in other people, it is energizing."

In fact, nurses who felt like they had the most control over their work and felt rewarded for it were the least likely to experience fatigue. Those nurses were also more motivated to work hard and do a good job.

A similar study found that certain temptations made students at Montreal's McGill University feel less motivated to work.

"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," said Michael Inzlicht - a University of Toronto psychologist who conducted the study. He added that there may be an evolutionary reason for this.

"As an organism, we need to meet multiple goals to survive. Because these multiple goals compete with one another [for our time], we need a mechanism in place that signals, 'Hey, stop doing that thing and do something else.'" 

He believes that fatigue may be the thing that tells you to switch gears.

And while Inzlich admits that he's still not entirely sure why desk jobs make us tired, he doesn't think it has anything to do with your lself control. Instead, it's likely you simply aren't motivated enough to work through the day.