Even one week of a high-sugar, high-fat, low-fiber diet can impair deep sleep. Research shows junk food reduces restorative delta waves, limiting overnight recovery—even when total sleep time stays the same.
In This Article:
- Why Sleep Quality Is Essential for Physical and Mental Health
- How Unhealthy Food Affects Sleep Quality
- Overview of the Swedish Study on Diet and Sleep
- How the Study on Junk Food and Sleep Was Conducted
- The Different Stages of Human Sleep Explained
- How a High-Sugar, High-Fat Diet Impairs Deep Sleep
- Health Consequences of Reduced Deep Sleep Quality
- Conclusion: Why Diet Quality Matters for Healthy Sleep
Why Sleep Quality Is Essential for Physical and Mental Health
Healthy sleep is enormously important for overall wellbeing. While we sleep, the body recovers from daily stress: cell damage is repaired, the immune system is activated, detoxification processes occur, and metabolism and hormones are regulated.
At the same time, the brain processes experiences from the day and forms new memory structures. When sleep is disturbed, these vital processes are disrupted—leading to a wide range of negative health effects.
How Unhealthy Food Affects Sleep Quality
A growing body of research suggests that what we eat directly affects how well we sleep. A study published in May 2023 by researchers from Uppsala University in Sweden examined how an unhealthy diet influences sleep duration and quality, with a particular focus on deep sleep. The findings were published in the journal Obesity.
Overview of the Swedish Study on Diet and Sleep
The study involved 15 healthy young men who:
- Were of normal weight
- Had regular sleep habits
- Slept between 7 and 9 hours per night
Researchers investigated how different diets influenced the structure and quality of participants’ sleep.
How the Study on Junk Food and Sleep Was Conducted
This was a crossover study, meaning each participant completed both dietary phases in random order:
- Healthier diet: Balanced nutrition with adequate fiber
- Western-style diet: Higher in sugar and saturated fat, lower in plant fiber
Key details:
- Meals were provided by the researchers
- Both diets contained the same number of calories, adjusted to individual needs
- Alcohol and caffeine were avoided
- Each diet lasted one week
- There was a seven-week break between phases
At the end of each phase, participants spent several nights in a sleep laboratory, where their sleep was closely monitored.
The Different Stages of Human Sleep Explained
Human sleep occurs in repeating cycles lasting about 90–110 minutes. These cycles consist of distinct sleep phases, each marked by specific brain wave patterns measured using an electroencephalogram (EEG) (1).
REM Sleep
REM (rapid eye movement) sleep features:
- Rapid eye movements
- Increased brain activity
- Intense dreaming
This phase plays a key role in emotional processing and memory formation (2).
Non-REM Sleep
Non-REM sleep is divided into three stages:
- Falling asleep
- Light sleep
- Deep sleep
During deep sleep, slow delta waves dominate. This phase is essential for physical recovery, immune strength, and tissue regeneration (3).
How a High-Sugar, High-Fat Diet Impairs Deep Sleep
After just one week on the high-sugar, high-fat, low-fiber Western diet, participants showed clear impairments in deep sleep quality:
- Fewer delta waves overall
- Reduced intensity of delta waves
These changes indicate a diminished capacity for overnight regeneration after consuming junk food (4).
Notably, the effects were still detectable the following night—even after participants had returned to a healthier diet.
Interestingly, total sleep duration and the length of individual sleep stages did not change. Researchers suggest this may be due to the relatively short study period.
Health Consequences of Reduced Deep Sleep Quality
Deep sleep is critical for long-term health. Even brief disruptions to this phase can interfere with immune function, repair processes, and metabolic balance.
A reduced or disturbed deep sleep phase has been associated with an increased risk of death from a wide range of diseases.
Conclusion: Why Diet Quality Matters for Healthy Sleep
Even a short period of unhealthy eating can measurably reduce sleep quality—particularly deep sleep (5).
While you may still sleep the same number of hours, your body’s ability to fully recover overnight is compromised.
Supporting deep, restorative sleep starts with daily choices.
Nutrition matters— not only for waking health, but for what happens while you sleep.
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