Too much light in your bedroom at night could raise your chances of developing type 2 diabetes, a major new study has found.
Even dim night-lights during sleep can disrupt the body’s natural daily rhythms, eventually leading to diabetes, a condition where blood sugar levels are poorly regulated. Bright lights before bed-time could also contribute.
While doctors already suspected a link between diabetes and artificial lighting, the new findings are the strongest evidence yet that excess night-time lighting is a real health risk, according to the team behind the study published in The Lancet Regional Health – Europe.
Professor Sean Cain at Flinders University in Bedford Park, Australia, said: “This is the largest ever study showing that real-world personal light exposure at night leads to increased diabetes.”
The results also suggest ways to lower your diabetes risk – including making your bedroom as dark as possible overnight, and limiting bright lights in the evening. “It really points to an easy, cost-free way to be healthier,” Professor Cain added.
Previous research has also suggested this kind of effect from artificial light, but has been on a smaller scale, or used different methods. One recent such study found that one night of higher light levels could change responses to blood sugar, which might raise diabetes risk – but it didn’t follow them up in the long term to check whether they did develop more diabetes.
In the new study, nearly 85,000 people in the UK agreed to wear a light-sensing device on their wrist for a week, which measured their surrounding light levels, day and night. Over the following eight years, those in the half of the group with the highest light levels at night-time – defined as between 12.30am and 6am – had about a 3 per cent chance of developing type 2 diabetes.
That was significantly higher than the diabetes rate of 2 per cent in the other half of the group, who had the lowest light exposure overnight.
That might seem like a small difference, but if a new medicine was developed that cut people’s risk of diabetes by that much, it would be “extraordinary”, said another member of the research team, Dr Andrew Phillips, also at Flinders University.
“This type of behavioural approach is better than a drug, because there are no side effects to reducing your night light,” he said. “Changing patterns of light exposure is a relatively cheap and straightforward thing that can be done.”
The scientists adjusted their figures to take account of income and whether people lived in towns and cities or the countryside. That was necessary because people who live in brightly lit urban centres might be more likely to have lower income levels and consequently worse health.
The increased risk of diabetes probably happens because special cells at the back of the eye detect changing light levels throughout the day and send signals to the hypothalamus, deep within the brain, which acts as a body clock, Dr Phillips said.
How to hack your body clock
There are several ways to reduce artificial lighting’s impact on your body clock, according to Dr Andrew Phillips at Flinders University in Bedford Park, Australia.
The most obvious is to avoid keeping any lights on in the bedroom overnight, even dim night lights. Thick “blackout” curtains or blinds can cut the illumination that gets in from street lights. If these steps aren’t possible, a sleep mask can be a workaround.
People can also try to avoid bright lighting in the last few hours before they sleep. Blue wavelengths of light are particularly stimulating, so that means don’t use phones or computers without turning on their “night-time” settings, when blue tones are reduced.
Some newer home lighting systems can be programmed to become less blue in the evening. “If you’re using lamps or overhead lights, you want them to be of a warmer yellower tone,” said Dr Phillips.
People may also want to reconsider late-night sessions at brightly lit gyms, especially as exercise can also make people feel more alert. An alternative approach to keeping the body clock on track is to boost light exposure in the morning.
“Bright light in the morning is absolutely great because it’s re-establishing a more natural circadian pattern,” said Professor Glen Jeffery at University College London.
The body clock controls many daily biological rhythms, including raising daytime levels of insulin, a hormone that helps keep blood sugar levels within a narrow range.
But if light is artificially high at night that can disrupt those rhythms, potentially leading to unhealthy levels of blood sugar.
“There are certain times of day when the body is anticipating feeding, and there are other times of day, such as when you would be asleep, that you’re not expecting any food, and so the body prepares for that in terms of its glucose and insulin responses,” said Dr Phillips.
“When the rhythms of that schedule are disrupted, we’re not responding to meals in the optimal way.”
Other research has shown that the light-sensing cells in the eye are especially responsive to blue wavelengths of light, potentially leading to even greater disruptions to the body clock.
Blue light is present in ordinary daylight, but is more intense in many electronic devices such as phones, computers and tablets. That makes it especially important to limit use of such devices at night, or turn on their “night-time” settings, which reduce blue tones, according to Professor Glen Jeffery at University College London, who was not involved in the latest study.
The new findings are not definitive proof that night-time lights cause diabetes, though. To find out for sure, we would need a trial to see if reducing light levels lowers rates of diabetes, said Dr Lucy Chambers of the charity Diabetes UK.
“The reasons why type 2 diabetes develops are multiple and complex,” she said. “Long periods of exposure to blue light has been identified as a possible contributing factor, but whether minimising this can meaningfully reduce risk of type 2 diabetes remains to be investigated.”
Lifestyle changes that have been proven to reduce diabetes risk include long-term weight loss, through diet and exercise, said Dr Chambers.