Remember your mother's favorite admonitions about watching television? "Don't sit too close—you'll rot your brain." "Junk food for the mind!" Granted, these days we aren't pressing our nose up to the living room tube as much as staring at smartphones, tablets, and laptops. We binge on entire seasons of Scandal or House of Cards, are glued to a thriller at 3 A.M. while our partner snores a foot away, and scroll through Facebook during bouts of insomnia. And it turns out Mom's warnings carry over: A couple of hours of up-close screen time right before bed—and often in bed—may be (a) messing with your sleep, and (b) causing you to pack on the pounds.

According to a recent study at the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, New York, people who spent two hours using a device with a backlit LED display—such as an iPhone or an iPad—had a corresponding dip in melatonin levels. "Melatonin is the 'darkness hormone,'" says lead researcher Mariana Figueiro. An uptick in melatonin production, which usually kicks in about two hours before your natural bedtime, preps you for sleep—telling the body it's time to nod off, she says. When the eyes are exposed to too much bright light at the "wrong" time of day, which is the case when you've got a screen six inches from your face late at night, it interferes with the suprachiasmatic nucleus, the part of the brain that regulates your body clock, or circadian rhythms, says sleep specialist Michael Breus, author of The Sleep Doctor's Diet Plan. So while it may feel cozy to get caught up on Instagram as you snuggle in, prolonged viewing can cut into your sleep or make it difficult to fall asleep at all. Worse, some experts theorize that late-night tech time may also be to blame for those impossible-to-lose 10 pounds: Additional research has found a significant link between sleep loss and weight gain. In a study at the University of Colorado at Boulder, participants who slept five hours a night for five nights consumed more calories, especially after-dinner snacks, than those who slept for nine hours. "If you're awake longer and later, it leaves you more time to eat, and often your choices are not the best quality at those hours," says physiologist Kenneth Wright Jr., who conducted the study. The researchers also found that those who don't get enough sleep tend to eat a smaller breakfast but then make up for it—and then some—come evening. And there's a cascade of hormonal changes that pile on when you're skimping on sleep, says Breus. "We know that sleep loss causes levels of the 'I'm hungry' hormone, ghrelin, to rise, while leptin, the 'stop eating!' hormone, dips," he explains. "At the same time we see that the stress hormone cortisol increases, which further stokes your appetite." So yes, you need to be mindful of how you use your iPhone and other devices. Here, some tips to ensure that your screen time doesn't affect your waistline.

WATCH THE CLOCK Limit the amount of time you're staring at a screen before bed—it's when you head north of 90 minutes that circadian rhythm disruptions can occur, Breus says.

GET SCREEN SAVVY Adjust the settings on your devices, advises Figueiro. "Dim the screen or tweak the contrast so that less light reaches the eye," she says, adding that Facebook and Outlook are notoriously bright and high contrast. On an e-reader, "reverse the background so you're getting white type on a black background, which can help."

CHECK OUT Avoid any nighttime screen activity that's too active—be it rapid-fire texting with a friend or getting amped up on Angry Birds. "It isn't passive, like watching TV," notes Breus. (He's the rare sleep doc who says it's okay to have a television in the bedroom, precisely because it's passive—and farther from your eyes.) The bottom line: It's still bikini season. Choose The Tonight Show over Twitter.

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