This is the greatest mystery of my adult life: How can I spend all day typing at a computer and go home feeling exhausted? How could merely activating the small muscles of my fingers leave me craving the couch at the end of the day?

This question actually lies very close to one of the more hotly contested issues in psychology: What causes mental fatigue? Why is desk work so depleting?

“It is kind of a mystery, to be honest,” said Michael Inzlicht, a University of Toronto psychologist who studies self-control, motivation, and fatigue.

But scientists do have some clues. There are two main hypotheses for why we get so tired from work when we’re not physically active. Let’s dive in.

Hypothesis 1: we get so tired because we deplete an internal store of energy

One hypothesis is resource depletion. That is, throughout the day, we draw on a limited store of mental energy. Some people call this willpower or self-control: the forceful use of mental energy to get at a goal. When our willpower stores get used up, we get tired. The analogy here is like a tank of gas; when it’s empty, the tank sputters out.

This hypothesis is called ego depletion, and it makes intuitive sense.

But the problem is, increasingly, psychologists aren’t sure it’s real. The basic ego depletion effect is that drawing on self-control to complete hard task drains us, but that wasn’t found in a recent 23-lab replication effort. Also, critics of the hypothesis argue, it doesn’t make much physiological sense.

One study estimated that a hardworking brain struggling with self-control may barely draw on the energy equivalent of a fraction of a single Tic-Tac compared to a brain at rest. Most of our caloric energy expenditure, as Vox’s Julia Belluz explains, goes into the background work of keeping our hearts, brains, and other organs running.

“‘Does your brain’s energy expenditure go up when you’re doing a hard math problem compared to when you’re zoning out watching TV?’ And everyone who has measured that has said ‘no,’” Kevin Hall, an obesity researcher at the National Institutes of Health, recently told her.

Overall, we have a bad intuition about how our brains and bodies use energy. And it doesn’t look like ego depletion is the answer to this vexing question.

Hypothesis 2: we get so tired because our motivation runs out

So if the resource depletion model of fatigue — gas running out — doesn’t make much sense, what does?

The other hypothesis from psychology involves motivation. That as we work on a task, we struggle to focus on it or eventually lose interest in it. We become less motivated to do the task. We become drawn to the things we want to do (scrolling Instagram or reading music blogs, for instance), rather than the things we have to do. And this tension possibly causes fatigue.

In August, researchers in the UK published new evidence that finds some indirect evidence for the motivational model.

This study tracked 100 nurses in the UK over two 12-hour shifts. Throughout the shifts, the nurses reported how fatigued they felt at regular intervals. They also wore devices that monitored and tracked the amount of physical activity they were engaged in. On average, the more hours the nurses worked, the more fatigued they felt. But when the researchers investigated what could possibly explain the fatigue, they found some interesting patterns.

Here’s the topline result: There was no correlation between the amount of physical work the nurses did and their feelings of fatigue. “In some people, physical activity is fatiguing,” Derek Johnston, the Aberdeen University psychologist who led the study, said. “But in other people, it is energizing.” The study also found that the nurses’ subjective sense of how demanding their job was of them was not correlated with fatigue either.

Instead, they found this small correlation: The nurses who were least likely to feel fatigued from their work also felt the most in control of their work, and the most rewarded for it. These feelings may have boosted their motivation, which may have boosted their perception of having energy.

Inzlicht has also found evidence for the motivational model in his work. A few years ago, he and Carleton University psychologist Marina Milyavskaya monitored 159 students at McGill University in Canada for a week. Throughout the week, the participants were peppered with text message questions about what temptations, desires, and effortful self-control they were engaging in at the moment, and whether they felt drained.

“What was surprising to us was the biggest predictor [of fatigue] was not whether they had exerted self-control [i.e., effort],” Inzlicht said. Instead, the predictor was the number of temptations 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 said. Temptations make us less motivated to do our work, which, in turn, may make us tired.

And there may be an evolutionary reason for why our brains would do this.

“As an organism, we need to meet multiple goals to survive,” Inzlicht explains. We’re not solely focused on finding food or finding mates, sleeping, or pursuing our passions in life. We need to do all these things to be a healthy, thriving species. “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.’” That mechanism, he suggests, could be fatigue.

In this light, boosting our motivation to stay on a task could lead us to feel less fatigued. One study found that just paying people some money when they’re depleted can keep them on task. A similar thing is found in studies on physical endurance: People can be easily pushed to work beyond what they think is their physical limit.

Why we need to figure out fatigue

As mentioned, psychologists don’t have this all sorted out. For one, it’s hard to track people, their motivations, desires, and fatigue throughout the day. Smartphone technology and Fitbit-like activity trackers are making it easier to track people in a dense, data-driven way. But there would need to be more research with larger samples to show, for sure, that fatigue is a motivation problem.

Learning about fatigue matters. When we’re fatigued, we’re prone to careless — perhaps dangerous — errors. Work is less enjoyable. And overall, fatigue is just not a nice feeling. The more we learn about fatigue, the better we can design safe, fulfilling work environments. This is helpful for bosses too: How can they best set up situations to make workers feel energized, motivated, and productive throughout the day? At the risk of becoming fatigued, I’m going to stop typing now.