Too much light at night, and not enough light in the day, could increase your risk of dying earlier than you otherwise would – a reminder of the importance of sleep health and of following the nudgings of our circadian rhythms.
From working night shifts or staying up late watching television, to being hidden away in offices or homes during the day, the habits and routines of modern life tend to disrupt a more natural relationship with light and dark.
And that's potentially deadly, according to the international team of researchers behind the new study. It adds to what we already know about the relationship between artificial light and a variety of health risks.
"Exposure to brighter nights and darker days can disrupt our circadian rhythms, a disruption that is known to lead to various health issues including diabetes, obesity, cardiovascular disease, mental health issues and increase the risk of death," says the study's senior author and sleep scientist Sean Cain, from Flinders University in Australia.
"These new insights into the potential adverse impact of light have shown us just how important personal light exposure patterns are for your health."
The researchers looked at records in a UK research database, pulling out information on 88,905 people who had worn light sensor trackers for a week, with their health then followed for around eight years.
Here are the raw stats: exposure to high levels of light at night was linked to a 21–34 percent increased risk of death, while exposure to high levels of daylight during the day was linked to a 17–34 percent lower risk of death.
A study like this isn't enough to prove direct cause and effect – there are too many other potential contributing factors to consider – but the association is strong enough to suggest light cycles are related in some way to health and mortality rate.
"Protection of lighting environments may be especially important in those at risk for both circadian disruption and mortality, such as in intensive care or aged-care settings," says lead author and sleep scientist Daniel Windred, from Flinders University.
Further computer modeling showed that the strength and rhythm of the participants' circadian rhythms – likely influenced by light exposure patterns – was also related to mortality rate, backing up the idea that it's the disruption to the body's natural flow that increases the risk of serious disease, and from there an earlier death.
While it's disconcerting to learn that this aspect of our modern lifestyles could be ushering us into an early grave, the good news is that the problem isn't too difficult to address: getting more daylight and avoiding light at night is something we can all try.
"Our findings clearly show that avoiding night light and seeking daylight may promote optimal health and longevity, and this recommendation is easy, accessible and cost-effective," says sleep scientist Andrew Phillips, from Flinders University.
The research has been published in PNAS.