Being afraid of the dark as a kid is common and as a result some people get used to sleeping with a light on. Continuing to do so as you get older may not be such a good idea when it comes to your health as a new study shows even moderate light exposure at night can stress the heart and increase insulin resistance.

Researchers from Northwestern University wanted to see what effect light has on the body during sleep based on the understanding light exposure during the daytime activates the sympathetic nervous system which heightens alertness and revs up the heart. They published their findings in the Proceedings of the National Academy of Sciences.

"Our results indicate that a similar effect is also present when exposure to light occurs during nighttime sleep," said senior study author Dr. Phyllis Zee said. "The results from this study demonstrate that just a single night of exposure to moderate room lighting during sleep can impair glucose and cardiovascular regulation, which are risk factors for heart disease, diabetes and metabolic syndrome. It's important for people to avoid or minimize the amount of light exposure during sleep."

Research indicated when the body is exposed to light at night it is unable to rest properly as it should.

"We showed your heart rate increases when you sleep in a moderately lit room," said study co-author Dr. Daniela Grimaldi. "Even though you are asleep, your autonomic nervous system is activated. That's bad. Usually, your heart rate together with other cardiovascular parameters are lower at night and higher during the day."

Scientists looked at the difference between moderate light (100 lux) compared to a dim light (3 lux) when it came to a single night of sleep. The moderate exposure caused the body to be in a higher state of alert during the night. That meant an increased heart rate as well as an increased force of contractions and faster blood flow.

The study participants who slept in the lighter room experienced insulin resistance the next morning. That meant the cells in their muscles, fat and liver were unable to respond as well to insulin and couldn’t use glucose from their blood for energy as effectively. As a result their bodies had to produce more insulin. That can cause blood sugar to go up over time.

Those findings mirror another recent study conducted with overweight participants when it came to light exposure at night.

"Now we are showing a mechanism that might be fundamental to explain why this happens," Zee said. "We show it's affecting your ability to regulate glucose. In addition to sleep, nutrition and exercise, light exposure during the daytime is an important factor for health, but during the night we show that even modest intensity of light can impair measures of heart and endocrine health."

Click here to read more in the Proceedings of the National Academy of Sciences.