In a recent study published in the journal adiposity, researchers tested 15 healthy young men of normal weight by having them try two separate diets for a week each. First, they were tested for their sleeping habits, all of which were normal, averaging seven to nine hours a night.

The researchers then conducted a randomized trial in which participants were given two separate diets: one with higher amounts of sugar, saturated fat and processed foods. The second, healthier diet contained much less fat and sugar and no processed foods.

The number of calories in both diets was the same and adapted to the daily needs of each person. Regarding timing, participants ate within a certain number of hours before going to bed.

During the week, each individual followed the prescribed diet as sleep quality, activity and eating schedule were monitored. After each diet, the participants were examined in a sleep laboratory. First, they had a normal night’s sleep, and brain activity was measured during rest. On the second night, they were kept awake, and then allowed to catch up on sleep. After that, their sleep was measured again.

“We saw that participants slept the same amount of time when following both diets,” said Jonathan Cedernais, MD, one of the study’s lead researchers. “Furthermore, across the two diets, participants spent the same amount of time in different stages of sleep.”

The difference, however, was in quality sleep “We were particularly interested in studying the properties of their deep sleep,” Sederne adds. “In particular, we looked at slow-wave activity, a measure that may reflect how restorative deep sleep is. Interestingly, we saw that deep sleep showed less slow-wave activity when participants ate junk food compared to eating healthier food. This effect also lasted until two in the morning when we switched the participants to an identical diet.”

So, even though they got the same amount of sleep and both recorded deep sleep, the participants who ate the unhealthy diet had less deep sleep and were therefore not as rested the next day as those who ate the healthier diet.