Researchers have found the brightness of a room matters more than you think.

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It’s no secret that sticking to your healthy diet is tougher when you eat out. There are the massive restaurant portion sizes, and sneaky calorie bombs (buffalo chicken salad, we're looking at you!)—and now new research suggests ambient lighting can trick you into eating more too.

For the study, published in The Journal of Marketing Research, the researchers began by looking at the orders of 160 diners at four chain restaurants. Half the diners were seated in dimly-lit rooms, and half were seated in well-lit areas. The diners who were sitting in low light ordered 39% more calories. Meanwhile the people in brighter surroundings were 16% to 24% more likely to choose healthy menu items (think grilled fish or white meat and veggies). In follow-up experiments, the researchers replicated these results with hundreds of colleged-aged students.

What is it about brightness that leads us to eat smarter? "We feel more alert in brighter rooms and therefore tend to make more healthful, forward-thinking decisions," lead author Dipayan Biswas, PhD, explained in a press release.

In fact, the researchers found that when diners in dimly-lit rooms were given a coffee placebo (or simply asked to be more alert) they were just as likely as their peers in the well-lit rooms to make healthy food choices.

It seems making healthy choices has more to do with the brightness of your mental state than the brightness of the room. So don't swear off candle-lit dinners yet.

The best way to avoid overindulging is to make yourself feel more alert, said co-author Brian Wansink, PhD, the director of the Cornell Food and Brand Lab.

That could mean going for a brisk walk before dinner, or splashing some water on your face for an instant energy boost. Pick-me-ups like these might actually be preferable, because as Wansink pointed out, mood lighting isn't all bad. In prior research, his lab has found that people who eat in darkened rooms enjoy their food more, eat slower, and consume less overall.