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
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