Moderate Light Exposure at Night Enough to Worsen Heart Health and Insulin Resistance

New research from Northwestern University suggests that being exposed to only slight ambient lighting while sleeping at night can harm heart health and fuel insulin resistance.

Using phone in the Dark

Findings from this study are of note because numerous people have their nighttime sleep while being exposed to light. People are exposed to nighttime lighting from numerous sources, including light-emitting devices such as television sets and mobile devices.

Researchers estimate that up to 40 percent of the population sleeps with some form of light on. This is a considerable number of people at risk, going by this new study.

Light exposure, even minimal, can impair cardiovascular function while sleeping and raise insulin resistance the morning after, the research revealed. This means an increased risk of heart disease as well as diabetes and metabolic syndrome.

The study has been published in the journal PNAS.

Effects of light exposure differ

Light does not have the same effects on a person during the day and at night. We need it to function well during the daytime but the reverse is the case during nighttime.

The human body has sympathetic and parasympathetic nervous systems for effective physiologic regulation round the clock. The sympathetic system is in charge during the daytime while the parasympathetic takes control during the night.

There is evidence that light exposure during the day activates the sympathetic nervous system. This results in increased heart rate, which helps us to be alert and active during the day.

The new research shed more light on what happens with the parasympathetic nervous system at night from light exposure.

Harmful health effects

The research team tested and compared the effects of sleeping at night with moderate light (100 lux) and dim light (3 lux). Subjects were monitored while they were asleep.

Moderate light, researchers observed, increased the participants’ alertness while they were sleeping. What this means is that the subjects exposed to it had a higher heart rate than those who slept in dimly-lit settings.

An elevated alert state increases the force of heart contraction as well as the speed with which the blood is carried to vessels for oxygenated blood supply.

“Even though you are asleep, your autonomic nervous system is activated,” explained co-first study author Dr. Daniela Grimaldi, a neurology research assistant professor at Northwestern. “That’s bad. Usually, your heart rate together with other cardiovascular parameters are lower at night and higher during the day.”

In addition, researchers noted a spike in insulin resistance the following morning after subjects slept while exposed to nighttime light. This increases a person’s risk of becoming obese and developing diabetes.

With insulin resistance, cells in the body fail to respond well to the hormone insulin that aids in controlling blood sugar. Glucose cannot make its way into the cells to be used for energy, therefore building up in the blood. The pancreas makes more insulin to check blood sugar but with minimal effects.

Scientists had shown in a previous study that people who were exposed to light during nighttime sleep tend to be more overweight and obese. That research appeared in JAMA Internal Medicine.

Dr. Phyllis Zee, the senior author of the current study, said their research showed how these outcomes were possible. This is by way of interference with glucose regulation in the body.

“These findings are important, particularly for those living in modern societies where exposure to indoor and outdoor nighttime light is increasingly widespread,” she said.

Zee, the sleep medicine chief at Northwestern’s Feinberg School of Medicine, emphasized the importance of avoiding or minimizing nighttime light exposure. There is likely too much light in your room if you can see things very clearly at night, she noted.