Humans spend nearly a third of their lives asleep, but this time is anything but wasted. Sleep is not passive downtime. It is an essential process that restores the body, regulates hormones, and protects the brain. When sleep is disrupted or consistently poor, the brain begins to show consequences that may not be immediately obvious but accumulate over time.
This is according to a comprehensive brain imaging study from Karolinska Institutet, published in the journal eBioMedicine.
A new study conducted in the United Kingdom involving more than 27,000 adults aged between 40 and 70 has shown a striking link between poor sleep and accelerated brain ageing. Researchers found that people with poor sleep habits had brains that looked significantly older than their actual chronological age, pointing to measurable effects on long-term brain health.
What it means when the brain looks older
While chronological age increases at the same pace for everyone, biological ageing can vary. Some individuals’ bodies and brains remain younger for longer, while others age more quickly. Recent advances in medical imaging and artificial intelligence now allow researchers to estimate a person’s “brain age” using detailed MRI scans.
Brain age is determined by analysing over a thousand features from scans, including the loss of brain tissue, thinning of the cortex, and damage to blood vessels. In this study, scientists first trained a machine learning model on scans from the healthiest participants, those without major diseases and whose brains closely matched their chronological age. The model then estimated brain age across the wider population.
A higher brain age than chronological age is considered a warning sign of accelerated decline. Previous research has linked older-appearing brains to cognitive decline, a higher risk of dementia, and even premature death.
Measuring sleep health across five characteristics
Because sleep is complex, no single measure captures its full impact on health. The researchers therefore considered five characteristics self-reported by participants:
• Chronotype, whether people identified as morning or evening types
• Sleep duration, with seven to eight hours considered optimal
• Insomnia symptoms
• Snoring, a possible indicator of sleep apnoea
• Daytime sleepiness
These traits often overlap. For example, a person who struggles with insomnia may also feel excessively tired during the day, while an evening chronotype may naturally sleep fewer hours. To create a more complete measure, the study combined the five traits into a single “healthy sleep score.”
Participants with four or five positive traits were classified as having a healthy sleep profile. Those with two to three were considered intermediate, while those with zero or one trait fell into the poor sleep category.
The impact of poor sleep on brain age
When brain age was compared across different sleep profiles, clear differences appeared. For every one-point decrease in sleep score, the gap between brain and chronological age widened by about six months.
On average, people with a poor sleep profile had brains that appeared nearly one year older than expected. Those with healthy sleep profiles showed no such gap.
Among the five sleep traits, late chronotype and abnormal sleep duration had the strongest associations with accelerated brain ageing. While a year may sound small, in the context of brain health it is significant. Even modest accelerations can accumulate over decades, increasing risks of dementia and neurological decline.
Why sleep affects brain health
The researchers explored several explanations for why poor sleep accelerates brain ageing. One is inflammation. Growing evidence shows that disrupted sleep raises levels of inflammatory markers in the body. Inflammation in turn damages blood vessels, triggers toxic protein build-up, and speeds up the death of brain cells.
Blood samples taken from participants confirmed this theory. Inflammation accounted for around ten percent of the link between poor sleep and brain ageing. Another explanation involves the glymphatic system, a waste clearance network in the brain that is most active during sleep. When sleep is insufficient or fragmented, this system may not clear toxins efficiently, allowing harmful substances to accumulate.
Poor sleep also contributes to other health conditions such as obesity, type 2 diabetes, and cardiovascular disease, all of which are independently damaging for brain health.
Lifestyle changes offer hope
The good news is that sleep is a modifiable factor. While some sleep disorders require medical treatment, everyday habits also play a major role in sleep quality. Experts recommend keeping a consistent sleep schedule, limiting caffeine and alcohol intake, reducing screen time before bed, and maintaining a dark, quiet, and cool environment for rest.
Addressing sleep apnoea and chronic insomnia with professional help can further reduce risks. By adopting healthier routines, individuals may slow the biological clock of their brains and lower their chances of developing dementia.
Broader implications for public health
The findings go beyond individual health. Sleep problems are widespread, with studies estimating that up to one-third of adults struggle with chronic sleep issues. If poor sleep accelerates brain ageing, it could significantly affect global health systems already preparing for rising rates of dementia.
Worldwide, dementia cases are expected to triple by 2050, placing enormous strain on families, caregivers, and healthcare infrastructure. Research linking sleep to measurable differences in brain ageing provides strong evidence for including sleep quality in public health strategies.
Global commentary and reactions
Experts not directly involved in the research welcomed the scale of the study. With more than 27,000 participants and a multidimensional measure of sleep health, the findings offer unusually robust evidence. Sleep researchers noted that linking sleep to an observable marker of ageing strengthens the case for treating sleep as a major pillar of preventive healthcare.
One neuroscientist commented that while brain ageing is inevitable, the pace at which it occurs is strongly influenced by behaviour. The study shows that improving sleep quality is not simply about feeling more rested, but about protecting the brain against long-term decline.
The path forward
The study’s findings underline a simple but powerful message. Brain ageing cannot be avoided, but its course can be influenced by choices made in daily life. Sleep is not just a period of rest but an active process with lifelong consequences.
Even small adjustments in habits can accumulate into meaningful benefits over decades. With populations living longer than ever, prioritising sleep may prove one of the most effective strategies for preserving brain health and reducing dementia risk.
As the researchers concluded, sleep deserves recognition as a fundamental component of healthy ageing. Making it a priority now can help keep the brain younger, sharper, and more resilient for years to come.
