Constant exposure to artificial light may have a number of negative health consequences, scientists have warned. They say that to be in good shape, switching off the lights at regular intervals may be as important as eating healthy food or exercising frequently.

Previous studies have indicated that light pollution and the use of artificial lighting around the world at night disrupts our circadian rhythm − our 24-hour cycle of physiological processes − and is a risk factor for bone deterioration.

This recent study, published in Current Biology and conducted on mice, comes up with similar findings, suggesting that prolonged exposure to artificial light can lead to pro-inflammatory activation of the immune system, muscle loss and early signs of osteoporosis.

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"Our study shows that the environmental light-dark cycle is important for health," says lead author Johanna Meijer of Leiden University Medical Center in the Netherlands.

"We showed that the absence of environmental rhythms leads to severe disruption of a wide variety of health parameters."

Mice exposed to light

The scientists worked with mice in their lab, exposing them to light around the clock for 24 weeks and then assessing several major health parameters.

They first measured the brain activity and found out that constant light exposure had an impact on the suprachiasmatic nuclei – the brain cells that control the circadian cycles. The normal rhythmic patterns in the suprachiasmatic nuclei were indeed reduced by 70% after prolonged light exposure.

light pollution sky
Light pollution seen on the horizon is created by artificial lights that cause 75% of the world's population not to experience full darkness at night B Hamann/360cities.net via Getty Images

Such a disruption to the circadian rhythm was correlated to a reduction in the animals' skeletal muscle function. Their bones showed signs of deterioration and the animals started developing osteoporosis.

The animals also entered a pro-inflammatory state normally observed only in the presence of pathogens or other harmful stimuli. This frailty is normally only observed in ageing animals.

The scientists thus say light exposure matters when it comes to thinking about the global health of populations.They will now investigate the influence of distorted light-dark cycles on the immune system.

Reversal can be achieved

It is however possible to reverse this process. The scientists worked to improve the mice's health condition by restoring a normal environmental dark-light cycle. After the mice were returned to this cycle for two weeks, the suprachiasmatic nuclei neurons rapidly recovered their normal rhythm and the animals' health problems were progressively reversed.

With 75% of the world's population exposed to light during the night and the prevalence of shift work remaining high around the globe, artificial light could become a serious global health problem. The fact that it could simply be avoided by switching off the lights to restore a normal circadian rhythm is therefore the good news of this study.