Exposure to nighttime light can throw off your body clock and sleep quality. And its hormone-disrupting effects may be why studies are now linking it with the health problems below. Fortunately, we've found some easy fixes.

1. Obesity. In a new study from the American Journal of Epidemiology, researchers at the University of Oxford in the U.K.found that women who slept in lighter rooms were more likely to have a higher BMI and larger waist size than women who slept in darkened rooms. Even dim light—about twice the brightness of a night light—suppresses production of the sleep hormone melatonin, which could mess with metabolism and eating patterns. If necessary, wear a light-blocking eye mask while you sleep, and avoid turning on the lights if you wake during the night.

2. Depression. Depressed people have much higher levels of nighttime light in their bedrooms, found a study published in 2013 in the Journal of Affective Disorders. Melatonin also boosts your mood, so too much brightness dampens them both. Light's sleep-disrupting impact could also be to blame, since poor sleep has been associated with depression. A dose of sunshine (the more the better, with proper sun protection, of course) might balance out the risk: "In our previous study, daytime light exposure enhanced nocturnal melatonin secretion," says Kenji Obayashi, MD, PhD, a researcher and professor at the Nara Medical University School of Medicine in Japan.

3. Breast cancer. Researchers in a study published in the April 2013 issue of the International Journal of Health Geographics discovered higher rates among women who lived in "areas with high levels of urban development, artificial lighting of roadways, shopping centers and homes," says researcher Sarah E. Bauer, who co-authored the study while at the University of Georgia. The melatonin dip may up estrogen, promoting tumor growth. Installing blackout shades on bedroom windows can minimize your exposure and risk, says Bauer.

4. Type 2 Diabetes. In a study from the April 2014 issue of the journal Chronobiology International, Obayashi and his team found that diabetics had more exposure to bright light during the four hours before bed. When the amount of light was doubled, the rate jumped 51%! Impaired melatonin mucks up levels of hormones like glucose and leptin, which regulate appetite and glucose. "Use low-wattage bulbs and minimize TV and computer use at night," says Bauer. These and other electronics like smartphones give off blue light, which has been shown to suppress melatonin more than other light wavelengths.

5. Insomnia. The bright-light group fared worse yet again in another study: They were more likely to suffer from poor sleep quality, and quantity, researchers reported. The decreased melatonin reduces sleepiness. If you must get up in the middle of the night–say, to use the bathroom–"forgo the usual lights for a dim red nightlight," says Bauer. "Red light suppresses melatonin production less than other wavelengths."

6. Blood pressure. Findings of a study reported in the July 2014 issue of the journal Chronobiology International show that people exposed to excessive nighttime light had higher blood pressure than those who slept in a darkened room. Though researchers aren't clear on the reason, previous studies suggest that light at night stimulates the nervous system, raising blood pressure. Cover your alarm clock if it gives off bright light, and unplug all other electronics, since even their chargers often emit light.

The bottom line: Aim to sleep in total darkness, and dim the lights a few hours before bed.

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