Artificial light at night may cause obesity

...The present study compares three groups of lab mice. One was exposed to a "regular day" of 16 light hours and eight hours of dark. A second group was in continuous light for 24 hours and the third group was given regular light for 16 hours followed by eight hours of dimmed light. The three groups were placed in these conditions over the course of eight weeks and were given equal quantities of food.

The results show that the mice experiencing dimmed light and those exposed to 24 hours of light gained about 12 grams of body mass, while the mice exposed to the "regular day" only put on about eight grams of extra body mass - close to 50 percent less than the others.

The researchers observed that there was no difference in the amounts of food that all the mice ate, or in the extent of physical activity (monitored as locomotor activity). The lab tests also showed drastically reduced glucose tolerance in those mice exposed to LAN.

Another finding, which led to the second stage of the study, showed a significant difference between the groups in their timing of eating: The mice exposed to dimmed light ate 55% of their food during the "night" hours while those experiencing "regular days" ate only 36.5% of their food at night. This prompted the researchers to see whether this was the cause of the marked differences in body mass gain.

To do so, they gave the group of mice enjoying "regular days" and those exposed to dimmed light hours, three different options: Unlimited eating times; food only during the light hours; and food only during the "night" hours. There was no need to include the third group in this step of the research, as they did not have any "night" hours.

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Once eating was limited to daytime only (which is when mice normally eat their food) or nighttime only (when they do not normally eat), there was no difference in body mass weight gain between the groups.

Haim explains that the study strengthens earlier findings by other researchers showing that exposure to LAN interferes with the production of melatonin - a hormone produced in the pineal gland in the brain under dark conditions at night. This interference causes changes in the body's cyclical functions and is what caused the mice to eat at abnormal hours...