In deciphering the mysteries of human health, mice have been one of our greatest allies. They have demonstrated the antibacterial properties of penicillin and served as a model for exploring obesity. Now they are helping researchers understand the potential negative effects of artificial light at night, or ALAN.

Mounting epidemiological data shows an association of ALAN with cancer, obesity, depression and osteoporosis. Previous work has studied the effects of short-term light exposure on mice, but the researchers at the University of Leiden in the Netherlands investigated how mice fared after long-term exposure. After 24 weeks mice exposed to 24 hours of continuous light had higher blood sugar levels, weighed more and had decreased muscle grip and bone structure when compared to mice exposed to the habitual cycle of 12 hours of light and 12 hours of dark.

An environmental cycle with distinct dark and light hours is, as the Leiden group concluded, essential for the health of mice. But what about human health? Unlike the mice in the experiment, the majority of humans are not exposed to 24-hours of constant light. Still, 90% of the people living in industrialized societies are exposed to ALAN. And, as Dr. Sarah Chellappa from Harvard Medical School’s Division of Sleep Medicine points out, the main take-home message from the paper is that “environmental light needs to be considered in the daily routine of humans”.

With our myriad of light-emitting devices it may seem that light pollution is inescapable. But while mice health deteriorated under constant light, their health quickly bounced back when they were placed under a more natural light-dark cycle. As Dr. Chellappa explains, this is because “light is one of the strongest zeitgebers” or synchronizers of our 24-hour biological rhythms. This means that when the mice were returned to a habitual light-dark cycle their physiological and behavioural responses also returned to a 24-hour cycle, thus allowing their health to recover. Since this zeitgeber-y quality of light is prevalent throughout the animal kingdom the same bounce-back would be expected in humans.

In 2012, the American Medical Association (AMA) called artificial lighting a “man-made self-experiment” and the results in this paper further confirm the need to both understand the health effects of ALAN and turn off the lights.


Special thanks to Dr. Sarah Chellappa, a research fellow at Harvard Medical School’s Division of Sleep medicine for her extensive expert commentary and suggestions.

Original Paper

Lucassen et al. Environmental 24-hr Cycles are Essential for Health. (Nature) – open access

Further Reading

Rebecca Boyle’s The End of Night (Aeon)

Managing Correspondent

Fernanda Ferreira

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