Researchers have demonstrated that light exposure at night disrupts metabolism; creating an obesogenic environment which favours tumor growth.

Motivation behind the research

It has become very common to stay up late at night to study, work or just for the sake of leisure. This extended period results in excessive exposure to light at night, stimulating a conflicting signal to the biological clock. This structure in our brain organizes the temporal order in our physiology and behavior (circadian rhythms) indicating high and low activity moments (activity, food consumption, hormone secretion, etc.) in accordance to the external environment.

The ability of light at night to disrupt the circadian organization is a potent risk factor for health problems including metabolic syndrome, immune dysfunction and cancer development.

In 2007, the International Agency for Research on Cancer classified shift-work with circadian disruption as a probable carcinogen to humans. We have previously demonstrated that circadian disruption causes immune alterations and promotes metabolic disturbances leading to an obesogenic environment, which is an additional cause for tumor growth. In this study, we tried to unravel the mechanisms linking cancer development to light exposure at night. We have hypothesized that tumors would grow more due to an altered inflammatory response and obesogenic metabolism in the host.

The Discovery

We exposed rats to light at night (LL) for 5 weeks and compared them with control rats maintained under a regular light-dark cycle (LD). The experiment revealed that LL induced overweight and metabolic problems, including and exacerbated inflammatory response, in rats. The inoculation of tumor cells under the skin allows us to observe increased tumor volume in LL rats as compared with LD rats. This increased tumor growth was associated with high blood glucose levels and decreased tryglycerides levels in the circulation. More macrophages (immune cells) were recruited in the LL tumor and the microenvironment inside the tumor showed upregulation of genes involved in fatty acid synthesis, glucose uptake and tumor growth, suggesting that LL tumors rely on these processes in order to support their enhanced growth. In order to confirm that the metabolic condition of the rats induced by LL may be the promoting factor for tumor growth, we fed LD rats with a high caloric diet and evaluated the tumor growth. We observed that tumors in high caloric fed rats had a similar growth rate as tumors in LL conditions. Our data indicates that circadian disruption by LL provides a favorable condition for tumor growth by creating an obesogenic environment in the host.

Study Limitations

This study follows tumor development in a short-frame of time and therefore, the long term consequences of light exposure at night on tumor development remains to be studied.

The Future

Future work will focus on trying to generate strategies in order to ameliorate the disrupting effects of light at night on the circadian function. This is especially important since an increasing number of night workers, young adults and children are exposed to artificial light for extended hours of the night and therefore, may be at risk of developing health problems.

Research Article: Circadian disruption promotes tumor growth by anabolic host metabolism; experimental evidence in a rat model. BMC Cancer. 2017.