Why light pollution may be linked to greater risk of Alzheimer’s disease

Light pollution is a growing issue. Over the past 25 years, artificial lighting has surged by 49% worldwide. For perspective, the brightness of the night sky has risen by almost 10% per year over the last 12 years. Around 80% of the global population is now affected by light pollution.

This is bad news, as light pollution is associated with a range of health problems, such as sleep disorders, fatigue, anxiety and deprivation.

Light pollution may also significantly increase the risk of developing Alzheimer’s disease, a recent study has found.

Alzheimer’s disease has many known risk factors linked with a greater risk of developing the disease. These include health conditions such as heart disease, high blood pressure and diabetes, as well as lifestyle habits such as smoking and drinking alcohol. This latest study now provides convincing evidence that light pollution is another risk factor of Alzheimer’s disease.

To conduct their study, the researchers used Medicare records from between 2012 and 2018, alongside satellite-collected light pollution maps from Nasa to examine whether there was a link between outdoor nighttime light exposure and the prevalence of Alzheimer’s disease in the US. The data featured a mix of men and women, as well as people from different ethnic backgrounds.

The study showed that for people aged 65 and older, living in an area with a lot of nighttime light pollution was a stronger risk factor for Alzheimer’s disease than chronic kidney failure, depression, heart failure, alcohol abuse and obesity – risk factors which all have a strong correlation with Alzheimer’s disease. However, diabetes, high blood pressure and stroke were still more strongly associated with Alzheimer’s disease than light pollution was in people aged over 65.

Interestingly, in those under 65, the study revealed that greater exposure to nighttime light pollution was linked to a higher prevalence of Alzheimer’s than all the risk factors analysed. This effect was shown in both genders and all races – except Pacific Islanders.

This could suggest that younger people may be especially sensitive to the effects of light exposure at night, compared to older people.

Sleep and brain health

As with most studies, it’s important to recognise that this research isn’t without limitations. Some variable were not adjusted for in this study which may influence the results.

For instance, artificial indoor light was not measured – such as the light emitted from phones, TVs and other electronic screens. Such factors are also associated with sleep disturbances and could affect the outcomes.

Another factor which wasn’t adjusted for and could affect the results is the influence of air pollution. Light pollution is closely connected to air pollution – with brightly illuminated areas often being busier urban centres. These urban environments have a higher concentration of vehicles, industrial activity and waste disposal. This all leads to increased levels of harmful particulate matter – a factor which is associated with cognitive decline and dementia.

The study’s data was also only specific to the US population. Health and socioeconomic status differences were not analysed which are important factors of Alzheimer’s and could have had an influence.

Still, this isn’t the first study to show a link between nighttime light pollution and brain health.

Previous research has also shown a link between nighttime light pollution and dementia. Studies from China and Italy found that people living in areas that had high levels of light pollution had a higher risk of developing mild cognitive impairment and late-onset dementia. This latest study adds to the body of evidence, showing a link between light pollution and Alzheimer’s disease specifically.

Research has shown there’s a link between light pollution and Parkinsons’s disease as well.

The exact reason why light pollution at night is linked to greater risk of Alzheimer’s disease is unknown. But it may be related to the way light pollution can interrupt sleep.

The link may be connected to the effect light can have on our sleep. Photographer: Waldo Swiegers/Bloomberg via Getty Images

Sleep plays an important role in the development of dementia. Research shows poor sleep may impair the glymphatic system – a network of vessels in the brain that clear waste and help wash away the toxic proteins that are thought to build up and cause Alzheimer’s. It’s theorised that without enough sleep, these proteins aren’t washed away and continue to build up.

Exposure to light at night can also interrupt a person’s circadian rhythm. This is the brain’s internal clock which manages our sleep and wake cycles by responding to changes in light around us. Disrupting this cycle can trigger inflammation and increase a person’s susceptibility to illnesses. Such inflammation can promote an increase in amyloid, a toxic protein that is typically found in the brains of Alzheimer’s patients.

There may also be genetic factors in play. For instance, the APOE gene is linked with greater risk of developing early-onset Alzheimer’s. This gene can affect how the body deals with stress.

This suggests that people under 65 might be less equipped to handle stressors like light pollution, possibly because young people are more likely to carry the APOE gene than older people. This may potentially make them more vulnerable to its detrimental effects.

This research contributes to the growing body of evidence showing a link between light pollution and Alzheimer’s disease. And with the majority of the world’s population exposed to light pollution, this could become an important risk factor when it comes to Alzheimer’s disease.

While it’s concerning that light pollution may have this effect on our health, it’s a modifiable environmental factor – meaning it’s something that we can take charge of in order to reduce our risk of Alzheimer’s disease. Even a few simple adjustments at nighttime can help reduce exposure to light pollution – including wearing an eye mask, using blackout curtains and dimming lights. DM 

This story was first published in The Conversation. Rahul Sidhu is a PhD Candidate in Neuroscience at the University of Sheffield.