Sleeping with the light on might scare away monsters under the bed, but it could be linked to an increased risk of heart disease and diabetes, research suggests. Light is an important signal by which the body’s internal clock, which governs a host of biological processes from temperature to hormone release, is synchronised to the external cycle of day and night. A number of studies have revealed, however, that keeping the lights on at night could be problematic. Among them, research has suggested it is associated with obesity in women and type 2 diabetes among elderly people, while a recent study from researchers in the Netherlands suggested bright daytime lighting and low light at night could help people with pre-diabetes control their blood sugar levels. Now researchers in the US say they have found that people who are exposed to artificial light at night show worse glucose and cardiovascular regulation compared with those who slumbered in the dark, something they suggest is down to the body being kept more alert. “We found that light – even [a] modest amount – increases activation of the autonomic nervous system, which we postulate increased heart rate and decreased insulin sensitivity,” said Dr Phyllis Zee, co-author of the study from Northwestern University Feinberg School of Medicine. Writing in the Proceedings of the National Academy of Sciences, Zee and colleagues report how they studied the glucose tolerance and heart rate of 20 people over two nights, 10 of whom spent both nights sleeping in dim light, while the other 10 spent one night in a room with dim light and the next night in a room with overhead lighting at around 100 lux, which is about equivalent to an overcast day. The results suggest that while levels of melatonin – a sleep-promoting hormone produced by the body – was similar in both groups, the group that spent a night with the lights on had higher insulin resistance in the morning, higher heart rate and lower heart-rate variability. “Because we only studied one night and in a healthy group, we are unable to say if these are clinically significant,” said Zee. “However the change in insulin would be considered a physiologically significant change that may translate into risk for disease.” Prof Jonathan Cedernaes from Uppsala University in Sweden, who was not involved in the research, said it was not surprising that light during sleep could have such effects, although he cautioned the new study was small and warranted confirmation in other cohorts and other conditions. “Light is the strongest signal to our circadian pacemaker that controls rhythms in physiology and behaviour, including in metabolism,” he said, adding the team behind the new study have previously shown blue-enriched light may have a specific impact on our metabolism. Cedernaes said people should try to have a sleeping environment that is as dark as possible – particularly for those doing shift work. Prof Russell Foster, director of the Sir Jules Thorn Sleep and Circadian Neuroscience Institute at the University of Oxford, also said the findings reinforced advice that darkness during sleep is important. “This is a very interesting study, and although the numbers are fairly small and the mechanisms not fully understood, the findings are consistent with previous observations that light at night can increase physiological alertness, reduce deep sleep and increase the release of stress hormones via the sympathetic nervous system. Together, these will act to increase insulin resistance,” he said. Foster added that the study points the way for larger scale trials over multiple nights, adding it would also be interesting to see if the results held in older individuals. “It would be interesting to see if this is an acute effect of one night of light exposure, or whether metabolic effects accumulate or decline over time,” he said. “Perhaps some individuals might even adapt to night-time light exposure over time.”