Consuming an unhealthier diet is associated with a lower quality of deep sleep, according to a small-scale study.
Deep sleep, the third stage of sleep, repairs and restores necessary functions such as memory, muscle growth, and immunity.
Researchers at Uppsala University in Sweden analysed how junk food affects sleep. Healthy participants consumed an unhealthier as well as a healthier diet in a randomised order.
The study, recently published in the journal Obesity, shows that after eating the junk food, the quality of the participants' deep sleep deteriorated, compared with those who followed the healthier diet.
"Both poor diet and poor sleep increase the risk of several public health conditions," said Jonathan Cedernaes, Associate Professor at Uppsala University.
A total of 15 healthy normal-weight young men participated in two sessions of the study. Participants were first screened for aspects such as their sleep habits, which had to be normal and within the recommended range of seven to nine hours per night.
The participants were randomly given both a healthier diet and an unhealthier diet. The two diets contained the same number of calories, adjusted to each individual's daily requirements.
Among other things, the unhealthier diet contained a higher content of sugar and saturated fat and more processed food items. Each diet was consumed for a week, while the participants' sleep, activity and meal schedules were monitored at an individual level.
"What we saw was that the participants slept for the same amount of time when they consumed the two diets. This was the case both while they were following the diets, as well as after they had switched to another, identical diet," said Cedernaes.
The researchers looked at slow-wave activity, a measure that can reflects 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 consumption of healthier food," Cedernaes said.
This effect lasted into the second night, once participants switched to an identical diet. Essentially, the unhealthy diet resulted in shallower deep sleep, the scientist said.
Similar changes in sleep occur with aging and in conditions such as insomnia, the researchers said.
It can be hypothesised, from a sleep perspective, that greater importance should potentially be attached to diet in such conditions, they added.
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