Unhealthy late-night eating behaviors make people less productive at work the next day, according to a new study published in the Journal of Applied Psychology — but only if you feel guilty or ashamed about eating the junk food.

For the study, 97 participants with fulltime jobs filled out surveys three times a day — at 8 a.m., 6 p.m. and before bed at 9:30 p.m. — for 10 days. The survey included questions about their physical and mental health, what they ate and drank and what they accomplished at work.

Researchers classified "unhealthy eating" behaviors as times when participants: "ate too many junk foods after work," "had too many unhealthy snacks after work," "ate and drank excessively after work" and "had too many late-night snacks before going to bed."

Using the participants' diary results, they "tested the lagged effects of the prior evening's unhealthy eating on the next day's performance," the study authors wrote.

Those who fell into so-called "unhealthy eating" habits at night spent less time doing "helping behaviors" at work, for example, assisting a co-worker with a task that is not your responsibility. They also had more "withdrawal behaviors," basically means avoiding work-related situations while on the clock.

Why?

As one might expect, researchers found that group dealt with some physical pain in the morning, like headaches, stomachaches and diarrhea.

But the group also reported more emotional stress, like feeling guilt or shame about eating habits — and that made a difference.

The findings "suggest work performance is compromised only when you experience emotional and physical strains," Seonghee Cho, study author and assistant professor of psychology at North Carolina State University, tells CNBC Make It. In other words, some people didn't think twice about their snacking the day before, so it had no effect on their work output.

"That means, if you can somehow intervene early on to avoid those strains, you may not suffer from decreased performance necessarily," she says.

For example, you can remind yourself that a "bad eating choice" in the evening isn't the end of the world, Cho suggests. "It can be a sporadic thing, or it can help release stress from that day, which actually serves a good stress coping strategy," she says.

Indeed, other research has shown that there is a time and place for comfort foods: "Unhealthy eating behavior may serve as a short-term coping strategy to deal with work-related demands," the authors wrote.

Ultimately, people have very nuanced relationships to food. It's important to "be more forgiving and chilled out about your suboptimal choice of eating given that we cannot always uphold the best and the healthiest diet regime," Cho says.

If you're looking for some habits that will make you feel sharp, rested and ready to go above and beyond and work, you're better off prioritizing habits that help you relax and detach from work, like physical activity, good sleep and healthy eating behaviors, Cho says.

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