For this study, 15 healthy participants consumed either a healthy diet or an unhealthy diet in a randomized order. According to the researchers, after being on the unhealthy diet the quality of participant’s sleep deteriorated, compared to those on the healthier diet. The participants were involved in study sessions over several days of monitoring in a sleep lab, after first being screened for several factors including sleep habits to make sure they were within the normal average range of 7-9 hours per night.
Participants were given a healthier diet and an unhealthier diet to consume in random order over the study period. The two diets contained the same number of calories which was adjusted to the individual’s daily requirements. However, the unhealthier diet contained a higher content of sugar, fat, and more processed food items among other things. Each meal had to be consumed at individually adjusted times which were matched for each diet, and each diet was consumed for a week during which time sleep, activity, and meal schedules were monitored for each participant.
“Both poor diet and poor sleep increase the risk of several public health conditions. As what we eat is so important for our health, we thought it would be interesting to investigate whether some of the health effects of different diets could involve changes to our sleep. In this context, so-called intervention studies have so far been lacking; studies designed to allow the mechanistic effect of different diets on sleep to be isolated,” says Jonathan Cedernaes, Physician and Associate Professor in Medical Cell Biology at Uppsala University.
“For example, deep sleep can be affected by what we eat. But no study had previously investigated what happens if we consume an unhealthy diet and then compared it to quality of sleep after that same person follows a healthy diet. What is exciting in this context is that sleep is very dynamic. Our sleep consists of different stages with different functions, such as deep sleep which regulates hormonal release, for example. Furthermore, each sleep stage is hallmarked by different types of electrical activity in the brain. This regulates aspects such as how restorative sleep is, and differs across different brain regions. But the depth or integrity of the sleep stages can also be negatively affected by factors such as insomnia and ageing. Previously, it has not been investigated whether similar changes in our sleep stages can occur after exposure to different diets.”
After each diet the participants were examined in a sleep lab, first sleeping a normal night while brain activity was measured to monitor their sleep. Then they were kept awake in the sleep lab before being allowed to catch up on sleep while brain activities were also recorded.
“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. In addition, across the two diets, the participants spent the same amount of time in the different sleep stages. But we were particularly interested in investigating the properties of their deep sleep. 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 consumption of healthier food. This effect also lasted into a second night, once we had switched the participants to an identical diet. Essentially, the unhealthy diet resulted in shallower deep sleep. Of note, similar changes in sleep occur with ageing and in conditions such as insomnia. It can be hypothesised, from a sleep perspective, that greater importance should potentially be attached to diet in such conditions,” explains Cedernaes.
“It would also be interesting to conduct functional tests, for example to see whether memory function can be affected. This is regulated to a large extent by sleep. And it would be equally interesting to understand how long-lasting the observed effects may be. Currently, we do not know which substances in the unhealthier diet worsened the depth of deep sleep. As in our case, unhealthy diets often contain both higher proportions of saturated fat and sugar and a lower proportion of dietary fibre. It would be interesting to investigate whether there is a particular molecular factor that plays a greater role. Our dietary intervention was also quite short, and both the sugar and fat content could have been higher. It is possible that an even unhealthier diet would have had more pronounced effects on sleep,” notes Cedernaes.
