Persistent exposure to light at night may cause weight gain – even without
changing physical activity or eating more food – according to research on
animals by scientists at Ohio State University, in which University of Haifa
biology Prof. Dr. Abraham Haim took part.
The researchers believe the
light could disrupt levels of the hormone melatonin, which is involved in
Exposure to artificial light at night has in numerous other
studies been linked to a significantly higher risk in humans of prostate and
breast cancer; the explanation is that the production of melatonin – released by
the pineal gland in the brain during darkness – inhibits cancer cell
In addition, melatonin may disrupt the expression of clock
genes, which help control when animals feed and when they are active, according
to the Ohio study. Overall, the findings show another possible reason for the
obesity epidemic in Western countries.
“Light at night is an
environmental factor that may be contributing to the obesity epidemic in ways
that people don’t expect,” Ohio State neuroscientist Prof. Randy Nelson said.
“Societal obesity is correlated with a number of factors including the extent of
light exposure at night.”
Until now, researchers have thought prolonged
computer use and TV viewing were risk factors for overweight, because they are
associated with a lack of physical activity.
“It may be that people who
use the computer and watch the TV a lot at night may be eating at the wrong
times,” Nelson said.
“Clearly, maintaining body weight requires keeping
caloric intake low and physical activity high, but this environmental factor may
explain why some people who maintain good energy balance still gain weight,”
The team found that mice exposed to a relatively dim light
at night over eight weeks had a body mass gain that was about 50 percent more
than other mice that lived in a standard light-dark cycle.
appears in the online early edition of the Proceedings of the National Academy
of Sciences (PNAS) that was released on Monday.
Mice were housed in one
of three conditions: 24 hours of constant light, a standard light-dark cycle (16
hours of light at 150 lux, eight hours of dark), or 16 hours of daylight and
eight hours of dim light (about five lux of light).
“Although there were
no differences in activity levels or daily consumption of food, the mice that
lived with light at night were getting fatter than the others,” Ohio State
University doctoral student and lead author Laura Fonken said.
suggest that mice living with light at night eat at times they normally
wouldn’t. In one experiment, mice exposed to light at night – but that had food
availability restricted to normal eating times – gained no more weight than did
mice in a normal light-dark cycle.
“Something about light at night was
making the mice in our study want to eat at the wrong times to properly
metabolize their food,” said Nelson.
If these results are confirmed in
humans, it would suggest that late-night eating might be a particular risk
factor for obesity, Nelson said.
The researchers measured how much food
the mice ate and how much they moved around their cages daily by using an
infrared-beam-crossing system. By the end of the experiment, light-at-night mice
had gained about 12 grams of body mass, compared to eight grams for those in the
standard light-dark cycle.
Although the dim light-atnight mice didn’t eat
more than others, they did change when they ate, results showed. These mice are
nocturnal, so they would normally eat substantially more food at night. However,
the dim light-at-night mice ate 55 percent of their food during the daylight
hours, compared to only 36% in the mice living in a standard light-dark
Since the timing of eating seemed significant, the researchers
performed a second study, similar to the first, with one important difference –
instead of having food freely available at all times, food availability was
restricted to either the times when mice would normally be active or when they
would normally be at rest. In this experiment, mice exposed to the dim light at
night did not have a greater gain in body mass than did the others when their
food was restricted to times when they normally would be active.
restricted their food intake to times when they would normally eat, we didn’t
see the weight gain,” Fonken said. “This further adds to the evidence that the
timing of eating is critical to weight gain.”