Exposure to artificial light at night linked to weight gain in women

Falling asleep to late-night TV or sleeping with a bright nightlight on, may mess with women’s metabolisms leading to weight gain, even obesity, suggests a study published in the journal JAMA Internal Medicine. The study found that women who slept with a television or night light on in the room they sleep had an association of gaining 11 pounds over a five-year period.

One of the lead authors of the study pointed out that “evolutionarily we are supposed to be sleeping at night, in a dark place.  It’s much more important than people realize for a whole variety of health reasons.”

During the day when we are exposed to light and then exposed to darkness during the night, helps maintain our 24-hour body clock.  This helps to regulate metabolism, sleep-promoting hormones, blood pressure, and other bodily functions. More and more research suggests when this cycle is disrupted, it may contribute to poor health, increasing risks for high blood pressure, diabetes, depressions, and obesity.

Study on exposure to light at night

For the study, researchers analyzed health and lifestyle data on almost 44,000 U.S. women, ages 35 to 74.  These women were enrolled in an ongoing study looking to find causes of breast cancer. Each women participating in the study underwent medical exams and filled out health and lifestyle questionnaires.  The data came from a nationwide cohort study called the Sister Study that enrolled women between 2003 and 2009. The data included information on each woman’s sleeping habits, such as if she slept with a small nightlight or a television on and her body mass index. Body mass index or BMI, is derived from a person’s weight and height.  It can be used as a screening tool for body fatness and obesity risk. A normal healthy BMI is typically considered to be between 18.5 and 24.9; a BMI of 30 and higher is considered to be obese.

Each woman involved in the study self-reported sleeping habits which were placed into four categories: no light, small nightlight in the room, light outside of the room, and light or television on in the room. Women who reported more than one type of artificial light were categorized at the highest level of exposure. Women who slept with a mask on or reported no light while sleeping were classified as experiencing no artificial light exposure.

Over the five-year period of the length of this study, researchers looked at each woman’s sleeping habits and the link between her weight and obesity risk.

Results of the study

What was found from this study was women who slept with a TV or light on in the room had a BMI increase of at least 30% and a higher risk of being overweight or obese, compared with women who slept with no lights on. Researchers stated they were confident that the added weight gain was not from factors such as snacking at night since the analysis accounted for other variables that could have led to weight gain including diet, physical activity and sleep duration. It was also noted that similar results would be found in men.

Animal research and smaller studies in humans have linked prolonged light exposure with weight gain.  Exactly how and why is uncertain but scientists think disruption in release of hormones related to sleep and appetite may be involved.

Take away from this study

While this study points out important information on sleeping with lights on as opposed to being off, it does have some limitations. One limitation is that the results are only an association and did not prove a causal relationship. More research is needed to determine whether sleeping with lights on actually could cause weight gain. Another limitation is that the data was based on self-reporting and the women were not asked why they kept lights on while sleeping.

A main take away from this study however, is it highlights a behavior that can be easily changed to possibly help reduce the risk of gaining weight. It also highlights a common recommendation that TVs and other technology should be kept out of bedrooms in order to maintain good sleep hygiene. Bedrooms should be environments to induce healthy sleep.  All of us need to respect our sleep and respecting sleep means making a sleep environment devoid of any type of light ideally but especially artificial light. Properly timed light of exposure to daylight while awake and darkness when asleep should be considered a part of a healthy life style, just like regular exercise and good nutrition.