Many adults suffer from insomnia, often without any obvious reason why they struggle to fall or stay asleep.
But a first-ever population level study has just given us a big clue.
Scientists have reported a significant link between insomnia in older adults and the levels of light pollution in an area – suggesting there's a relationship between artificial light and how easily we can sleep soundly in our later years.
The new research pulled data from the population-based National Health Insurance Service-National Sample Cohort (NHIS-NSC) in South Korea during the years 2002-2013, using sleeping aid prescriptions as an indicator of having trouble sleeping.
Light exposure was recorded with satellite data, and mapped to residential areas. In total, data from 52,027 adults aged 60 years or above was used, with females making up about 60 percent of the participants.
Light pollution, mapped. (Journal of Clinical Sleep Medicine)
Not only were those in more light-polluted areas more likely to be taking these drugs, the study shows, on average they're more likely to have been using them for a longer time and at higher dosages.
"Our results are supportive data that outdoor, artificial, nighttime light could be linked to sleep deprivation among those while inside the house," says one of the researchers, Kyoung-bok Min, from Seoul National University College of Medicine in South Korea.
This doesn't prove once and for all light pollution causes or worsens insomnia – there might be other factors at play too, not covered by this study.
For example, the team suggests more light pollution might mean a more built-up area, which might mean easier access to sleeping drugs.
That said, the large sample size does suggest there's some kind of relationship here, and one we need to look at for the good of our health.
Previous studies show that sleep quality and duration is vital to our well-being for all kinds of reasons, from skin ageing to heart health. If we're not getting enough shut-eye, then our whole body suffers.
And this study isn't alone in linking light pollution with health issues. While artificial light has no doubt helped us as a civilisation, we're now seeing more evidence of the damage it can do too – it's even been linked with higher breast cancer rates.
We also know that light can cause disruption in our circadian rhythm – that built-in biological clock that helps us know when it's time for some serious rest. Staring at smartphones is thought to be one way light interferes with the body's rest patterns.
And if you need more of a reminder of the link between light and sleep, consider the study published earlier this year that showed how humans living underground can end up sleeping for days in some cases.
Now we might have to add artificial light from outdoors as a threat to our overall health too, so even if our phones are safely switched off, the glare of a streetlight or shop sign can be harmful to the regular patterns of wakefulness and sleepiness.
One issue the study doesn't cover in detail is how this outdoor light ends up affecting indoor sleepers. Some light might seep inside, the researchers suggest, or a lot of outdoor light might make people more likely to turn lights on indoors, if it seems too dark.
The study findings could perhaps be used to identify those at risk of insomnia in the future, its authors suggest.
"Given the recent scientific evidence including our results, bright outdoor lighting may be a novel risk factor for prescribing hypnotic drugs," says Min.
The research has been published in the Journal of Clinical Sleep Medicine.