"It's really important that you have that daytime-nighttime cycle, so that you appropriately regulate hormones, hormones that regulate your sleep, hormones that regulate your hunger, said lead author Dale Sandler, a scientist with the National Institute of Environmental Health Sciences, a division of NIH". The women were asked about their sleep habits and whether they slept in total darkness or in the presence of artificial light from a television or nightlight.
"It also may disturb day-to-day variations of stress hormones and affect other metabolic processes in ways that contribute to weight gain", Park added. The results suggest that cutting off lights at bedtime could reduce women's chances of becoming obese.
The researchers analyzed health and lifestyle data on almost 44,000 US women enrolled in an ongoing study seeking clues to causes of breast cancer.
Among the women, the researchers found that sleeping with a television or light on in the room was associated with gaining five kilograms or more, a BMI increase of at least 10%, and a higher risk of being overweight or obese, compared with being exposed to no artificial light during sleep. And the level of artificial light seemed to matter, Park said.
The researchers considered several other compounding factors, like sleep deprivation, which could have played a role in the association between artificial light exposure at night and weight gain.
Light coming in from outside the room was associated with more modest weight gain, researchers found.
Dr. David Katz, director of the Yale-Griffin Prevention Research Center in New Haven, Conn., reviewed the findings.
"Even with the lights off, our bedrooms are often aglow at night from luminous clocks, light-emitting diodes from electronic devices, and outside lighting that seeps through porous curtains and shades", Gangwisch said.
"As with any study of association, two findings are true - true, but not directly related", he said.
Yet the study findings appear to fall in line with separate research - including one study, published in the Journal of Clinical Endocrinology and Metabolism in 2016, that linked increased light exposure at night with a 10% increase in body mass index over a 10-year period in older adults.
More research is needed to better understand the link and determine whether reducing light at night may prevent obesity, the authors concluded.
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