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.”