The key to getting a good night’s sleep lies in the sleeping environment — a comfortable mattress, reduced noise, a 65- to 72-degree room temperature, and a dark room. For those who are afraid of the dark, a night light may be the best way to fall asleep, but it could wreak havoc on your health. According to a recent study published in the American Journal of Epidemiology, sleeping in a room with too much light may increase body mass index (BMI) and waist size for women.

"Metabolism is affected by cyclical rhythms within the body that relate to sleeping, waking and light exposure," said Professor Anthony Swerdlow from the Institute of Cancer Research in London, The Telegraph reported. Dating back since evolutionary beginnings, being active when it's light in the day and resting when it's dark at night is part of our human circadian (24-hour) body clock. Light has the ability to alter moods, physical strength, and even the way we process food in a 24-hour cycle. Disruption of sleep and circadian rhythms via light exposure could be a contributing factor to the rise in obesity, although it has not been investigated in humans.

Emily McFadden, co-author of the study and a visiting researcher at the institute, along with her colleagues, sought to investigate whether the disruption of circadian rhythms by exposure to light at night (LAN) could contribute to weight gain in women. The study, funded by Breakthrough Breast Cancer, was an attempt to understand the risk factors for breast cancer, which obesity is known to increase the odds of the disease. Data was drawn from the Breakthrough Generations Study, a cohort study of women aged 16 years or older who were recruited during 2003-2012.

The women were asked to rate the amount of light in their bedrooms at night as: light enough to read; light enough to see across the room, but not read; light enough to see your hand in front of you, but not across the room; and too dark to see your hand, or you wear a mask. Their answers were compared to several measures of obesity such as BMI, waist-to-hip ratio, and waist circumference.

The findings revealed the odds of obesity increased with increasing levels of LAN exposure. The associations found between LAN and obesity were very intriguing for the researchers. "In this very large group of people there is an association between reported light exposure at night and overweight and obesity,” Swerdlow told the BBC. However, he cautions there is not sufficient evidence to know if making your room darker would have any effect on your weight.

The researchers do suggest artificial light can contribute to weight gain since it is known to disrupt the body clock by delaying the production of melatonin — the sleep hormone. This study adds weight to previous studies that looked at animals into how light exposure, circadian rhythms, and metabolism could all be connected in some way. A 2010 study found persistent exposure to dim light at night affected the weight, body fat, and glucose intolerance (the underlying cause of late-onset diabetes) in mice. The researchers found the mice that lived with light at night were getting fatter than the others.

These studies highlight changes in metabolism could be caused by light at night since light disrupts levels of melatonin, which is involved in metabolism. This suggests light at night could be an environmental factor that may be contributing to the obesity epidemic — as more than one-third of U.S. adults are obese. Identifying artificial light sources at night and monitoring our use may give us a good night’s sleep and help us stay fit and healthy.


Ashworth A, Jones ME, McFadden E, Schoemaker MJ, Swerdlow AJ. The Relationship Between Obesity and Exposure to Light at Night: Cross-Sectional Analyses of Over 100,000 Women in the Breakthrough Generations Study. American Journal of Epidemiology. 2014.

Fonken LK, Haim A, Morris JS et al. Light at night increases body mass by shifting the time of food intake. PNAS. 2010.