Dying for a nap after lunch? You may want to consider swapping the potato chips for an actual potato.

A new study found that eating more fatty foods was linked to greater levels of sleepiness during the day. When people ate more carbohydrates, their alertness scores soared.

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For the study, researchers enlisted 31 adults who were healthy and not obese to spend four nights in a sleep lab. The researchers tracked their meals and tested them for sleepiness with a procedure called the Multiple Sleep Latency Test, which uses electrodes to measure brain waves, eye movements and other indicators.

Results, published recently as an abstract in the journal Sleep and to be presented next month at a meeting of the Associated Professional Sleep Societies, linked high-fat diets with more sleepiness. Higher-carb diets were linked with more alertness.

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Previous studies found the same associations between diet and how tired people subjectively report themselves to be. The new study is the first to provide objective evidence that high-fat diets may make it harder to get through the afternoon slump.

“Excessive daytime sleepiness and fatigue are very prevalent in the modern world and on the rise,” said psychiatrist Alexandros Vgontzas, of the Penn State College of Medicine, in a press release. “It appears that a diet high in fat decreases alertness acutely, and this may have an impact on an individual’s ability to function and also public safety.”