"Innocuous" exposure to light at night alters circadian rhythms, which may in turn lead to weight gain, a new review suggests.
Clues that this might be a problem were first found in mice, which, when exposed to even low levels of light at night, "would shift their timing of food intake, and even though they were not eating any more calories, they were still gaining weight," coauthor Laura Fonken, PhD, from Ohio State University, Columbus, told Medscape Medical News.
"We think these results could be important to humans as well, [because] we all use computers late at night. We leave lights on late at night," she continued. "We have streetlights that come in our windows. All these different sources of light may be affecting the circadian system, which has consequences for metabolism."
This "very interesting and very novel" review also looks at human epidemiologic studies, mostly in shift workers, said Endocrine Society spokesperson Eve Van Cauter, PhD, from the University of Chicago, Illinois, who was not involved in the study.
"Together, this body of evidence suggests that an additional environmental factor, which has not been taken into account so far in any of the explanations of the epidemics of obesity and diabetes, might be this increased exposure to light at night," she added.
"We don't [yet] have the kind of very strong evidence of a potential risk factor [for metabolic disease] that organizations like the American Diabetes Association would want to really implement new guidelines," Dr. Van Cauter continued.
"But this is so simple for practicing physicians to ask [their patients] about their exposure to nighttime light and suggest [ways to minimize it], and it makes so much sense, that it's not necessary to wait until the scientific and biomedical community has absorbed these messages….It's [also] very important that physicians ask their patients whether they are shift workers, because that influences the results [of certain blood] tests."
Could Nighttime Light Exposure Be Driving Obesity?
As night pollution—brightening of the night sky caused by streetlights and other artificial sources—in the United States has been increasing over the past few decades, the rate of obesity has also been rising in parallel.
The review examines the literature to see whether this is merely a coincidence or whether electricity and the ability to have "unnatural" self-selected sleep/wake schedules could be affecting health by disrupting the circadian system and in turn be associated with altered metabolism, perhaps contributing to the growing obesity epidemic.
Epidemiological evidence from shift workers suggests that prolonged exposure to light at night increases the risk of developing cancer, sleep disturbances, mood disorders, metabolic dysfunction, and cognitive impairments, Dr. Fonken and coauthor Randy J. Nelson, PhD, also from Ohio State University, write.
Circadian rhythms are implicated in energy regulation, they note. And multiple genes that are involved in nutrient metabolism display rhythmic oscillations, as do numerous hormones related to metabolism, such as glucagon, insulin, ghrelin, leptin, and cortisol.
In mouse experiments, those with mutations in circadian clock genes have altered feeding behavior, endocrine signaling, and dietary-fat absorption. In the human epidemiologic studies, shift workers were more likely to be obese and have hypertension and hypertriglyceridemia.
Three Pathways: Disturbed Circadian Rhythm, Hormones, and Sleep
The authors describe 3 pathways by which they theorize that light at night might increase the risk for weight gain. In addition to disrupting the circadian clock, it also disturbs cortisol and melatonin signaling and impairs sleep.
"Promoting awareness of circadian biology and the consequences of nighttime light exposure in both the scientific community and general public is critical," they note. People can minimize exposure to light at night by making simple changes, "such as using 'blackout' curtains or sleep masks to block out streetlights, turning off hallway lights, and removing televisions and computers from bedrooms," they add.
It is also important to keep a consistent sleep schedule to minimize "social jet lag," which can occur when a person wakes up early on weekdays for social reasons (to go to work or school) and then stays up late and sleeps in on weekends, Dr. Fonken said.
Future studies should examine nighttime light levels in different settings, including nursing homes and hospitals, where people may be particularly vulnerable, the researchers conclude.