Daily exposure to artificial lighting could cause significant health issues.
A study published in the Cell Press journal Current Biology finds that when mice are exposed to constant artificial lighting for over 5 months, they exhibit many detrimental side effects and a decline in quality of health.
Several of the vital processes and functions of the body follow a natural daily rhythm - the circadian clock - based on 24-hour day-night, light-dark cycles.
Circadian rhythms are controlled by the body's biological "clock." There is also a "master clock" in the brain that coordinates all the body clocks so that they are in synch.
The "master clock" consists of a group of nerve cells in the brain called the suprachiasmatic nucleus (SCN), which contains 20,000 neurons and is located deep within the brain in the hypothalamus.
Circadian rhythms control physical, mental and behavioral changes that follow a 24-hour cycle such as the sleep-wake cycle, body temperature, heart rate, and the release of hormones.
"Our study shows that the environmental light-dark cycle is important for health. We showed that the absence of environmental rhythms leads to severe disruption of a wide variety of health parameters."
Johanna Meijer, Leiden University Medical Center in the Netherlands
Health parameters that could be disrupted by a lack of environmental rhythm include pro-inflammatory activation of the immune system, muscle loss, and early signs of osteoporosis.
The mice in the study experienced negative physiological changes from the chronic artificial light exposure that is observed in aging animals. However, when the standard light-dark cycle was restored, the negative effects were reversed.
Meijer and colleagues examined the association between the light-dark cycle and disease by measuring various primary health parameters in mice after they had been exposed to artificial light continually for a total of 24 weeks.
Restoring light-dark cycle reverses negative effects on health
Brain activity studies uncovered that around-the-clock light exposure led to a reduction in the normal rhythmic patterns in the brain's central circadian pacemaker of the SCN by 70 percent.
Tests of strength in the mice revealed that when light and dark patterns were non-existent, and the circadian rhythm disrupted, there was a reduction in skeletal muscle function, bones deteriorated, and pro-inflammatory signs were observed.
Reinstating normal light-dark cycles for a 2-week period stimulated a recovery in the normal rhythmic patterns of the SCN and a reversal of the negative health issues in the mice.
"We used to think of light and darkness as harmless or neutral stimuli with respect to health," says Meijer. "We now realize this is not the case based on accumulating studies from laboratories all over the world, all pointing in the same direction. Possibly this is not surprising as life evolved under the constant pressure of the light-dark cycle."
"We seem to be optimized to live under these cycles, and the other side of the coin is that we are now affected by a lack of such cycles."
Future studies from the team will involve in-depth analysis of light-dark cycles on the immune system. Also, with the common exposure of patients to artificial light in hospitals, the researchers would like to investigate the possible health benefits to patients when they are exposed to normal light and dark cycles.
With 80 percent of the world's population living under light-polluted skies, light exposure and the effect it has on people - especially in those that are aging or otherwise vulnerable - are topics that need to be further studied.