There is a close link between lifestyle choices, such as diet and exercise, and a person's weight. Researchers now identify another factor as they report that sleeping with a light or television on puts women at higher risk of becoming overweight or obese.

How much a person weighs is down to a complex interplay of personal and societal factors. The rate of obesity has nearly tripled across the globe since the mid-1970s, and in the United States, an estimated 70% of the population are overweight or have obesity.

According to the Centers for Disease Control and Prevention (CDC), having obesity increases a person's risk of a range of adverse health conditions, including high blood pressure, abnormal cholesterol levels, type 2 diabetes, heart disease, stroke, osteoarthritis, some cancers, mental health problems, and all cause mortality.

Diet and exercise are the most highly studied factors that contribute to the rising obesity epidemic. Others, such as genetics and sleep, are emerging as additional players.

A new study, which recently featured in JAMA Internal Medicine, points to another culprit: artificial light in our sleep environment.

'Light or TV on in the room' linked to weight

Dale P. Sandler, who is Chief of the Epidemiology Branch and a senior investigator at the National Institute of Environmental Health Sciences, part of the National Institutes of Health (NIH), NC, and Dr. Yong-Moon Mark Park, a fellow working with Sandler, led the research.

The study involved 43,722 women who took part in the Sister Study, which looked at environmental risk factors and the chances of developing breast cancer.

Each participant provided details about her exposure to artificial light at night (ALAN) at the start of the study, allowing Park and his colleagues to divide the volunteers into four categories: no light, small night-light in the room, light outside the room, and light or television on in the room.

The research team had access to data that Sister Study professionals had collected at the start of the study, which included the women's weight, height, and hip and waist circumference. They also had the women's self-reported height and weight measurements at both the start and the follow-up, which occurred after an average of 5.7 years.

When the researchers analyzed the data, they identified a correlation between increasing levels of light exposure and health outcomes at the beginning of the study.

The more light a woman experienced in her sleeping environment, the higher her body mass index (BMI) and waist to hip ratio were, on average. On the flip side, sleep duration was shorter, and physical activity and healthful eating scores were markedly lower.

At the follow-up point, women who had a television or light on in the room had a 17% higher risk of gaining 5 kilograms (roughly 11 pounds) or more compared with women who reported no light while sleeping. They also had a 13% higher risk of a BMI increase of 10%. The risk of becoming overweight or obese was 22% and 33% higher respectively.

The associations held when Park took into consideration possible confounding factors, such as inadequate sleep, diet, and physical activity.

"The fact that the association between artificial light at night and weight gain remained after we controlled for sleep characteristics is important new information," Park commented to Medical News Today.

Yet, the authors point to the limitations of the study, which include the fact that light exposure was self-reported and that they did not take into consideration why women chose to sleep with lights on in the room or whether light exposure changed over time.

"While our study provides stronger evidence than other previous studies, it is still not conclusive. Even so, it seems reasonable to advise people not to sleep with lights on."

—Yong-Moon Mark Park

"Sleeping with electronic devices, lights, or TV on may be a hard habit to break. Others may have family members who are up later with the lights on," Park explained. "Those who live in crowded housing or in cities may have a harder time controlling the amount of light that comes in from outside the room."

Meanwhile, the team is planning on looking at how environmental light exposure affects weight and whether there are links between artificial light and other conditions, such as high blood pressure and type 2 diabetes.