Finally, a study proves that potato chips can help you get lean: Cheat days may help you stick to your healthy eating plan in the long term, new research from Portugal suggests.

Scientists put study participants on a diet that was limited to only 10,500 calories per week. (That’s compared to the average American man’s weekly intake of 17,969 calories, according to the U.S. Department of Agriculture.)

But half of the dieters were allowed to eat anything they wanted on Sundays.

After two weeks, the two groups had both reduced their average body mass indexes. While there was no significant difference in the amount of weight the two groups had lost, those who had taken Sundays off were happier and more motivated to continue working toward their weight-loss goals.

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The reason: Occasional indulgence makes dieting more sustainable, the researchers say.

Self-restraint wears you out, and if you’re drained and grumpy, you’re more likely to give up. But if you can look forward to a “free day” where you enjoy items that wouldn’t normally fall within your healthful eating plan, you’ll be more likely to stick to your diet the rest of the week.

The key is to plan ahead and designate a specific day for your rule-breaking, says study author Rita Coelho do Vale, Ph.D. That’s because giving in to a spur-of-the-moment donut can make you feel like you blew your diet—and might as well abandon it completely.

Related: 6 Sinister Reasons You Overeat

But don’t confuse “cheat day” with “binge day.” The study participants kept their portions of pizza, ice cream, and other treats within reason so they could stay within their weekly calorie totals.

The bottom line: Don’t swear off certain foods completely. Go ahead and enjoy those Doritos every Friday. They may just help you finally move the needle on the scale.

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