Eating a diet that is high in fat and sugar may reduce sleep quality by messing with the brain’s electrical activity during deep sleep.
When we go to sleep, our brain’s electrical activity slows down. The higher-frequency brainwaves that dominate while we are awake, called beta waves, are gradually replaced by lower-frequency ones called delta waves.
The deepest, most restorative stage of sleep – called slow-wave sleep – has the highest proportion of delta waves. It typically occurs in the first half of the night and allows the body to repair itself and consolidate memories.
Jonathan Cedernaes at Uppsala University in Sweden and his colleagues tested the effects of a Western-style high-fat, high-sugar diet on slow-wave sleep in 15 men with an average age of 23.
The men were randomly assigned to eat a high-fat, high-sugar diet or a low-fat, low-sugar diet for one week. They then slept for one night in a laboratory wearing an electroencephalography (EEG) cap to record their brain’s electrical activity. After a break of several weeks, they switched to the other diet and repeated the laboratory sleep study.
The participants were provided with all their meals and had to eat them at set times. The high-fat, high-sugar diet included foods like sweetened granola, pizza and chocolate, whereas the …
low-fat, low-sugar diet contained foods like unsweetened muesli, salmon and vegetables. The two diets were matched for calories.
Each man typically slept for the same length of time on the two diets and felt their sleep quality was the same.
However, the EEG recordings showed that the high-fat, high-sugar diet disrupted slow-wave sleep by reducing the proportion of delta waves and increasing beta waves, suggesting it was less restful. This happened in 11 out of the 14 men for whom full results were collected.
The effect may be because sugar and fat activate brain pathways that increase how awake people feel, but more work is needed to unravel the mechanisms, says Cedernaes. He also hopes to repeat the study to find out if the change also occurs in women.
We don’t know the long-term impact of disrupting slow-wave sleep in this way, but we do know that poor diets typically lead to worse health, which may partly be explained by their effect on sleep quality, he says.
The amount of slow-wave sleep that we get naturally declines with age, so eating a healthy diet may be particularly important in older age to protect against further losses in sleep quality, says Cedernaes.