The investigators, from the Departments of Neuroscience and Psychology, Ohio State University, Columbus, Ohio, and the Israeli Center for Interdisciplinary Research in Chronobiology, University of Haifa, explain as background information that the steady rise in the rates of obesity and metabolic disorders among humans have coincided with an increase in LAN and shirt work.
The 24-hour rhythm that regulates the state of our internal energy levels and metabolism is controlled by an internal biological clock that works in parallel with and responds to light information, the authors wrote. Our internal body clock ("circadian clock") prepares us for predictable events, such as the availability of food and sleep. When the function of this clock is disturbed, our bodies experience a disruption in our metabolism and body-rhythms (circadian cycle).
Put simply - the predictability of light and day regulates our body clock, which regulates our metabolism. When the light and dark cycle is disrupted, so is our body metabolism, and also when we decide to eat.
The investigators assessed the effects of LAN on the BMI (body mass index) of male mice to see whether there might be a casual relationship between exposure to light during the night and obesity.
They found that mice exposed to light at night had considerably higher BMIs and lower glucose tolerance compared to mice kept in a normal day/night cycle (dark at night). The difference in BMI persisted even after their calorie intakes and total daily physical activity were altered.
Laura Fonken, lead author, said:
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.
They also found that eating times were different. Mice are by nature nocturnal eaters - they consume more calories in darkness than during the daylight hours. The researchers found that the mice kept in an LAN environment (dim light at night) consumed over half their food during the daylight hours, compared to the mice in the natural light-dark environment which consumed a just over a third of their calories during daytime.
Co-author, Randy Nelson said:
Something about light at night was making the mice in our study want to eat at the wrong times to properly metabolize their food.
Nelson added that if humans respond to light cycles in a similar way to mice, and if these results could be confirmed in humans, it may mean that late-night eating is a major obesity risk factor.
The authors concluded:
These results suggest that low levels of light at night disrupt the timing of food intake and other metabolic signals, leading to excess weight gain. These data are relevant to the coincidence between increasing use of light at night and obesity in humans.
When we restricted their (the mice's) food intake to times when they would normally eat, we didn't see the weight gain. This further adds to the evidence that the timing of eating is critical to weight gain.
Corticosterone - is a stress hormone we have often linked to metabolism changes in humans and consequent weight gain. In this study the researchers found no significant difference in corticosterone levels between the mice in different light-cycle environments, demonstrating that for metabolism changes in mice, corticosterone levels do not need to change.
Melatonin - this is a hormone produced by the pineal gland which is closely involved in regulating the sleep/wake cycles of animals, including humans. The authors believe that levels of melatonin, which is also involved in metabolism, are disrupted when light exposure becomes unpredictable and disrupted. This could also disrupt the expression of clock genes which help animals regulate their activity and feeding times. Nelson said:
Light at night is an environmental factor that may be contributing to the obesity epidemic in ways that people don't expect. Societal obesity is correlated with a number of factors including the extent of light exposure at night.
Our linking how long we spend on computers or watching TV to weight gain may perhaps have been too narrowly associated with a lack of physical activity. It seems highly likely that other factors are also at play, the authors suggest.
It may be that people who use the computer and watch the TV a lot at night may be eating at the wrong times, disrupting their metabolism. 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.
"Light at night increases body mass by shifting the time of food intake"
Laura K. Fonkena, Joanna L. Workmanb, James C. Waltona, Zachary M. Weila, John S. Morrisb, Abraham Haimc, Randy J. Nelson
PNAS Published online before print October 11, 2010, doi: 10.1073/pnas.1008734107