In recent years, studies have found that diet, stress, a lack of sleep, obesity, and gut inflammation can all increase Alzheimer's disease risk in some individuals.

Now it appears that artificial outdoor light overnight could be a factor too, especially for those under 65 years of age.

Researchers from the Rush University Medical Center in Illinois compared light pollution maps across 48 US states to medical records for the same areas, finding a statistically significant link between light pollution and Alzheimer's.

"We show that in the US there is a positive association between Alzheimer's disease prevalence and exposure to light at night, particularly in those under the age of 65," says medical scientist Robin Voigt-Zuwala.

"Nightly light pollution – a modifiable environmental factor – may be an important risk factor for Alzheimer's disease."

For the over-65s, light pollution was ahead of risk factors such as alcohol abuse, chronic kidney disease, and depression. However, it was still behind other risk factors, including diabetes and high blood pressure.

That changed for people aged 65 and under: here light pollution was the number one Alzheimer's risk factor out of those assessed in the study. The indication is that younger people are even more susceptible to the damage caused by light at night.

It's not clear exactly what's behind this link – which doesn't prove direct cause and effect – but we already know how important sleep and circadian rhythms are to good health, and how disruptive artificial light can potentially be to those routines.

"Certain genotypes, which influence early-onset Alzheimer's disease, impact the response to biological stressors which could account for increased vulnerability to the effects of nighttime light exposure," says Voigt-Zuwala.

"Additionally, younger people are more likely to live in urban areas and have lifestyles that may increase exposure to light at night."

Sleep is vital for growing brains, and has previously been linked to the risk of dementia and other diseases.

Since the advent of electric light in the 19th century, the trend for increasing levels of artificial light has only gone in one direction, and there's clear evidence that it's interfering with our sleeping patterns.

The findings are a reminder of the potential cost of our modern day lifestyles, to balance against all the benefits nighttime light brings us. It's thought that more than 80 percent of the world's population is affected by light pollution.

"Awareness of the association should empower people – particularly those with risk factors for Alzheimer's disease – to make easy lifestyle changes," says Voigt-Zuwala.

"Easy to implement changes include using blackout curtains or sleeping with eye masks. This is useful especially for those living in areas with high light pollution."

The research has been published in Frontiers in Neuroscience.