According to a new working paper from Harvard Business School, setting aside 15 minutes to write at the end of the workday is enough to make you better at your job.

"When people have the opportunity to reflect, they experience a boost in self-efficacy," says HBS professor Francesca Gino. "They feel more confident that they can achieve things. As a result, they put more effort into what they're doing and what they learn."

In a series of laboratory and field experiments, Gino and her colleagues found that reflection leads to better performance. The most telling example comes from their field experiment with an Indian outsourcing company called Wipro.

The researchers put new employees into groups where people either reflected on their days or didn't. In the reflection group, employees were given a paper journal and asked to spend 15 minutes at the end of their workdays writing about what went well that day, which they did for 10 days.

The result: The journaling employees had 22.8% higher performance than the control group.

"In the field study, we were asking people to work less," Gino says. "It's counterintuitive, because you think you want to use those 15 minutes to keep working, but it actually leads to performance."

The study sprang from the experiences Gino and her colleagues have had as instructors. After each class she teaches, she takes time to debrief, noting the comments students made and what points of hers led to quality discussions. She then folds those insights into the next class she teaches.

That daily routine is easy to replicate across industries. It's just a matter of making an appointment with yourself to reflect on the day's successes so you can incorporate those lessons into the next day.

It's like the process of "iteration" that startup folks are always talking about. You introduce a stimulus, gather the data of your experience, and then improve from there.

But you shouldn't be keeping the learning in your head, Gino cautions. Take the extra step and write it down.

Psychology research shows that writing about your life experiences has loads of positive effects, including increasing student's grade point averages, re-employment after losing a job, and improving memory. This is because writing helps "codify" the things you've gone through, Gino says.

"It's very easy to deceive yourself if you're just thinking about it," she adds, "but when you write things down on paper, it's easier to identify what's helpful."