People
exposed to earlier sunlight are leaner than those who get afternoon light
A
surprising new strategy for managing your weight? Bright morning light.
A new Northwestern
Medicine® study reports the timing, intensity and duration of your light
exposure during the day is linked to your weight -- the first time this has
been shown.
People who had most
of their daily exposure to even moderately bright light in the morning had a
significantly lower body mass index (BMI) than those who had most of their
light exposure later in the day, the study found. (BMI is a ratio calculated
from a person's weight and height.)
"The earlier
this light exposure occurred during the day, the lower individuals' body mass
index," said co-lead author Kathryn Reid, research associate professor of
neurology at Northwestern University Feinberg School of Medicine. "The
later the hour of moderately bright light exposure, the higher a person's
BMI."
The influence of
morning light exposure on body weight was independent of an individual's
physical activity level, caloric intake, sleep timing, age or season. It
accounted for about 20 percent of a person's BMI.
"Light is the
most potent agent to synchronize your internal body clock that regulates circadian
rhythms, which in turn also regulate energy balance," said study senior
author Phyllis C. Zee, M.D. "The message is that you should get more
bright light between 8 a.m. and noon." About 20 to 30 minutes of morning
light is enough to affect BMI.
Zee is the Benjamin
and Virginia T. Boshes Professor of Neurology and director of the Northwestern
Medicine Sleep and Circadian Rhythms Research Program at Northwestern
University Feinberg School of Medicine. She also is a neurologist at
Northwestern Memorial Hospital.
"If a person
doesn't get sufficient light at the appropriate time of day, it could
de-synchronize your internal body clock, which is known to alter metabolism and
can lead to weight gain," Zee said. The exact mechanism of how light
affects body fat requires further research, she noted.
The study will be
published April 2 in the journal PLOS
ONE. Giovanni Santostasi, a
research fellow in neurology at Feinberg, also is a co-lead author.
What's the
Magic Number for Low BMI?
Many people do not
get enough natural light in the morning, Zee said, because the American
lifestyle is predominantly indoors. We also work in poorly lit environments,
usually about 200 to 300 lux. In the study, 500 lux was "the magic
number" or minimum threshold for having a lower BMI. Even on a cloudy day,
outdoor light is more than 1,000 lux of brightness. It is difficult to achieve
this light level with usual indoor lighting, the scientists noted.
Light May
Be Next Frontier for Weight Loss
"Light is a
modifiable factor with the potential to be used in weight management
programs," Reid said. "Just like people are trying to get more sleep
to help them lose weight, perhaps manipulating light is another way to lose
weight."
Santostasi, a
physicist by training, developed a new measure for the study that integrates
the timing, duration and intensity of light exposure into a single number
called mean light timing or MLiT.
He searched for a
correlation between light exposure timing, duration or intensity in the study
raw data, but none of those factors individually were associated with BMI. It
was only when he began combining parameters, that he saw "the strong
signal" when all three were examined together.
"I saw that
what seemed to be most associated with body mass index was not just how much
light you receive but when you get it and for how long," Santostasi said.
The study included
54 participants (26 males, 28 females), an average age of 30. They wore a wrist
actigraphy monitor that measured their light exposure and sleep parameters for
seven days in normal-living conditions. Their caloric intake was determined
from seven days of food logs.
Befriend
Your Body Clock – Get Morning Light
The finding
emphasizes the importance of "circadian health" in which exposure to
light and dark is synchronized with your internal body clock. "We focus on
how too much light at night is bad; it's also bad not to get enough light at
the appropriate time during the day," Zee said.
As part of a
healthy lifestyle, people should be encouraged to get more appropriate exposure
to light. Workplaces and schools should have windows. Employees should be
encouraged to go outside for lunch or breaks, and indoor lighting should be
improved in the school and workplace "This is something we could institute
early on in our schools to prevent obesity on a larger scale," Zee said.
While duration and
timing of sleep was not linked to the results, "owl" chronotypes, who
stay up later and sleep later, would be a population affected by later light
exposure. But even "larks," those who wake early, would be affected
by lack of early light if they stayed inside in the morning.
While
the study wasn't designed to examine how light exposure affects body fat,
previous research at Northwestern and elsewhere shows light plays a role in
regulating metabolism, hunger and satiety.
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