A new study suggests that a proper day-night light cycle is critical for good health. In the study, just published in Current Biology, mice exposed to continuous light 24 hours a day developed health problems. When they were returned to a regular cycle of light and darkness, the mice recovered.
Researchers from the Leiden University Medical Center kept mice in an environment where they were exposed to constant light. The mice were kept like this for 24 weeks. The team then measured their brain activity and analyzed their general health. The researchers found that the mice exposed to constant light showed reduced activity in the brain’s suprachiasmatic nuclei, or SCN. The SCN is a group of brain cells responsible for various physiological and behavioral rhythm cycles that normally occur throughout the day and night. The SCN neurons control circadian rhythms in both mice and humans. In the experimental group, SCN activity was reduced by about 70%.
The group of mice exposed to continuous light also became weaker. They had reduced skeletal muscle function, performed poorly in strength tests, and their bones had started to deteriorate. The mice also appeared to enter a pro-inflammatory state that normally only occurs when an animal is severely stressed or exposed to disease.
The experimenters then switched the mice back to a regular day-night cycle. After 2 weeks, the mice were back to normal and all health problems had been successfully reversed. Their SCN neurons began to operate as normal and the mice showed no long-term symptoms.
These findings suggest that a normal cycle of light and darkness is critical for maintaining good health and proper SCN neuron activity. The authors note that constant exposure to light is common in human society, especially in settings such as hospitals and nursing homes. Many people also work third shift or live in an area where it’s difficult to sleep in darkness. The good news is that the negative effects of constant light exposure are easily reversed after switching back to a normal day-night cycle. The researchers now plan to study how improper light-dark cycles affect the immune system.
Lucassen et al. Environmental 24-hr Cycles Are Essential for Health. Current Biology (2016).