The Easiest Way to Keep Off Weight

Just 20 minutes of sunshine can boost your metabolism

Good news for your waistline: A shot of sunshine in the morning may lower your body mass index (BMI), according to a new study from Northwestern University’s Feinberg School of Medicine.

Regardless of an individual’s physical activity level, diet, sleep, or age, people who had most of their daily exposure to natural light between 8 a.m. and noon had significantly lower BMIs than those who got most of their exposure later in the day.

The reason: Natural light provides a boost to your circadian rhythms—or your body’s internal clock—which is known to regulate your metabolism, hunger, and energy levels, says co-lead author, Giovanni Santostasi, Ph.D. In fact, previous studies have found that constant exposure to bright light later in the afternoon or at night can throw off your circadian clock and increase your risk of obesity.

So how much sun do you need? The researchers discovered that 500 units of illuminance (lux) is the magic amount needed to affect your BMI—and morning outdoor light provides thousands of lux. Even a cloudy morning can provide 1,000 lux. Those who had more of their daily light exposure above 500 lux earlier in the day had a lower BMI than those who had the same level of light later in the day.

Artificial indoor light won’t cut it, either. Most offices are poorly lit, offering only 200 to 300 lux. “Any amount of outdoor light is better than staying inside all day,” says the study’s other lead author, Kathy Reid, Ph.D.

Along with timing and intensity, duration matters, too. You need to spend at least 20 to 30 minutes outside to affect BMI. Bring your morning coffee outside, read the newspaper by a large window, walk to your favorite lunch spot, take your dog for a jog, or go for a bike ride. “The longer you are in the light and the stronger the rays, the better your results,” she says.


The Easiest Way to Keep Off Weight