Junk food not only makes you fat - it stops you getting a good night's sleep as well, new research shows. A diet of pizza, take-aways and processed food impairs our deep sleep which can cause memory problems.
It also interferes with hormones which can lead to high blood pressure and obesity. Researchers at Uppsala University gave volunteers an unhealthy as well as a healthy diet in a randomised order.
After the unhealthy diet, the quality of their deep sleep had deteriorated, compared with those who had followed the healthy diet. Jonathan Cedernaes, Physician and Associate Professor in Medical Cell Biology at Uppsala University said: "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."
Previous studies have shown that diets with greater sugar content, for example, are linked to poorer sleep. Prof Cedernaes added: "Sleep is an interplay of different physiological states and 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.” For the study, published in the journal Obesity, 15 healthy normal-weight young men participated in two sessions over several days in a sleep lab.
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 of sleep per night. In random order, volunteers were 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. The meals of each diet had to be consumed at individually adjusted times, which were matched across the two dietary conditions.
Each diet was consumed for a week, while the participants' sleep, activity and meal schedules were monitored at an individual level. After each diet, the participants were examined in a sleep laboratory. There, they were first allowed to sleep a normal night, while their brain activity was measured to monitor their sleep.
They were then kept awake in the sleep laboratory, before being allowed to catch up on sleep. Their sleep was recorded in this case, too. Prof Cedernaes said: “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." The researchers do not currently know how long-lasting the sleep effects of the unhealthier diet may be and they did not investigate whether the shallower deep sleep may alter functions that are regulated by deep sleep.
Prof Cedernaes added: “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."
