A study recently published in the journal Current Biology has shown that long-term exposure to artificial lighting for over five months may impair circadian rhythms and have detrimental effects on health including a reduction in immune function and muscle strength, and bone deterioration.
Virtually all organisms have circadian rhythms — mental, behavioural and physical changes that follow a roughly 24-hour cycle (e.g., sleep-wake cycle, the release of hormones, heart rate, and body temperature), responding largely to light and darkness in an organism’s environment. Circadian rhythms help organisms foresee and adapt to the environmental day-night cycle and control several of the key bodily processes and functions. The body’s “biological clock” controls circadian rhythms and a “master clock” in the brain organizes all the body clocks so that they are in sync. The “master clock” is made up of a group of nerve cells in the brain called the suprachiasmatic nucleus (SCN), which is found within the hypothalamus of the brain. The SCN ultimately produces circadian rhythms that are synchronized in several bodily processes, including immune system function, muscle function, and bone metabolism.
Previous studies have shown that the pervasive use of artificial lighting in current society—especially during the night—disrupts circadian rhythms and can be harmful to health. To examine the association between light-dark cycle and disease, a team of researchers exposed mice to artificial light for 24 weeks, followed by 24 weeks in a normal light-dark cycle, and many major health parameters were measured. Continuous artificial light exposure was used as a model to induce disturbances to circadian rhythms. The researchers aimed to test whether continuous exposure to a light-dark cycle affects health (e.g. the immune system and metabolic function), and to determine whether these effects were able to be reversed. Rhythms in the central clock were measured through electrophysiological recordings in the SCN of mice. The researchers measured the influence of long-term artificial light on immune system function, bone microstructure, and skeletal muscle function at different points in time during and after 24 weeks of exposure to artificial light.
The findings, published in Current Biology, showed that 24 weeks of exposure to artificial light had significant harmful effects on a range of health parameters, as it led to a 70% decrease in normal rhythmic patterns in the central circadian pacemaker of the SCN. Long-term artificial light exposure (i.e. when environmental light-dark cycles were non-existent) also resulted in a significant reduction in skeletal muscle function (wire hanging duration, grid hanging duration, and forelimb grip strength), and bone deterioration characteristic of early osteoporosis, and induced temporary changes in the immune system, including a pro-inflammatory state. After a 2 week period during which mice had been returned to a normal light-dark cycle, rhythms in the SCN and the majority of health effects previous mentioned returned to normal.
Taken together, these findings suggest that exposure to artificial light (i.e. an absence of environmental rhythms or light-dark cycles) in the long term impairs circadian rhythms and plays a role in susceptibility to disease, as a disrupted circadian rhythm reversibly induces harmful effects on numerous biological processes. The researchers also point that exposure to long-term artificial light conditions often exists in nursing homes and intensive care settings, in which lighting can fluctuate so little throughout the 24-hr period that patients often fail to entrain to these light-dark cycles. For instance, in patients in intensive care, rhythms that control a variety of mental, behaviour, and physical changes are often disrupted, possibly due to an absence of environmental rhythms in these settings.
Overall, the study provides evidence that long term (24 weeks) exposure to artificial light and lack of environmental rhythms impairs circadian rhythms in the central clock, thereby reversibly reducing immune function, bone structure, and muscle strength, and supports that clear environmental light-dark cycle is important to maintain a healthy state. These findings produce new opportunities for prevention and treatment programs, especially for frail individuals, such as nursing home residents, intensive care patients, and the elderly. The findings are also applicable to large parts of the population, as 75% of the world’s population is exposed to artificial light during the night. The researchers suggest that long-term studies be carried out to study the health effects of increasing daily light levels (and decreasing light exposure during the night) in such settings. As well, the long-term effects of the environmental light-dark cycle on the immune system, bone microstructure, and muscle function in humans are currently unknown. The study provides strong evidence that the detrimental effects of chronic exposure to light warrant further examination.
Written By: Nigar Celep, BASc