In a recent study published in the journal Obesity, researchers tested 15 healthy young men of normal weight, making them try two separate diets for a week each. They were initially screened for their sleep habits, all of which were normal and within the range of seven to nine hours per night. 

After this, the researchers conducted a randomized trial in which participants were given two separate diets: One contained a higher amount of sugar, saturated fat, and processed foods. The second, healthier diet was much lower in fat and sugar and contained no processed foods.

The number of calories in both diets was the same and adjusted to each individual’s daily requirements. In terms of timing, participants ate meals within a consistent number of hours before bedtime.

Each individual ate their assigned diet for a week as their sleep quality, activity, and meal schedules were tracked. After each diet, the participants were examined in a sleep laboratory. First, they slept a normal night while having their brain activity measured during rest. On the second night, they were kept awake before being allowed to catch up on their sleep. After this, their sleep was measured once again. 

“What we saw was that the participants slept for the same amount of time when they consumed the two diets,” Jonathan Cedernaes, M.D., Ph.D., one of the lead researchers in this study, said in a statement. “In addition, across the two diets, the participants spent the same amount of time in the different sleep stages.”

The difference, however, was in the quality of sleep. “We were particularly interested in investigating the properties of their deep sleep,” adds Cedernaes. “Specifically, we looked at slow-wave activity, a measure that can reflect how restorative deep sleep is. Intriguingly, we saw that deep sleep exhibited less slow-wave activity when the participants had eaten junk food, compared with the consumption of healthier food. This effect also lasted into a second night once we had switched the participants to an identical diet.”

Therefore, even though they slept the same amount and both recorded deep sleep, the participants on the unhealthy diet had shallower deep sleep and therefore were not as rested the next day as those on a healthier diet.