Here's a reason to set that alarm a little earlier: Being exposed to sunlight early in the morning (instead of later in the day) is associated with a lower BMI, a new Northwestern University School of Medicine study reports. This association was found regardless of other factors like how many calories people ate or how much they exercised.

Early morning light exposure accounted for 20 percent of the variation in BMI among people studied. Light sends an extremely strong signal to your body and brain to wake up and also helps regulate your body's circadian clock, says Kathryn Reid, Ph.D., one of the study's authors. Your circadian rhythm may impact your metabolism too. Reid points out that studies show that animals with changes to their circadian rhythm gain weight even though they didn't eat more. Messing with your body clock, then, and missing out on early rays may "alter the way your body processes the foods you eat, leading to weight gain," she says. What's more, it may also affect your hunger and satiety signals.

You need about 20 to 30 minutes of morning sunlight to reap the benefits, Reid says. That's easy to get in the summer when you're eager to be outside, but in the winter? Not so much. While daylight is the most efficient source (and it's the brightest source of light), being close to a window indoors will work also. And if you can't be outside and your office has no windows, use desk lamps. Overhead light doesn't get into your eyes, which is how light enters your circadian system.