A study analyzing data from more than 13 million hours of light sensor data collected from 89,000 people has found that exposure to light nights and dark days is associated with an increased risk of mortality.

Bright light at night disrupts the body’s normal day-night cycle, known as the circadian rhythm, and can lead to insomnia and the development of serious diseases. Researchers set out to see if personal day and night light, as well as light patterns that disrupt our circadian rhythm, can predict the risk of death. The results, published in the journal Proceedings of the National Academy of Sciences, show that people exposed to more light at night had a 21-34% increased risk of death, while those exposed to more daylight had a 17-34% decreased risk of death.

Light Exposure and Risk of Death

“Exposure to brighter nights and darker days can disrupt our daily rhythms, a disruption that is known to lead to various health issues such as diabetes, obesity, cardiovascular disease and mental health problems, as well as increasing the risk of mortality,” says lead author and sleep expert Professor Sean Cain from Flinders University. These new findings about the possible negative effects of light have shown the researchers how important personal light exposure patterns are for health. Co-senior author, Associate Professor Andrew Phillips notes that nighttime light exposure disrupts circadian rhythms by shifting the timing (phase shift) and weakening the signal (amplitude suppression) of the central “circadian pacemaker” that controls circadian rhythms throughout the body.

Disruption of the body’s circadian rhythms is associated with the development of metabolic syndrome, diabetes and obesity, and is also strongly implicated in the development of cardiometabolic diseases such as heart attack, stroke and hypertension. The observed associations between nighttime light exposure and mortality risk may be explained by the fact that nighttime light disrupts circadian rhythms and leads to negative cardiometabolic consequences, according to the researchers. “Our results clearly show that avoiding nighttime light and taking advantage of daylight can promote health and longevity.”

The authors of the FHMRI Sleep Health study examined the relationship between personal light exposure and cardiometabolic and all-cause mortality risk in 89,000 UK Biobank participants aged 40 to 69. Readings were recorded using wrist-worn sensors, and participants’ mortality was recorded by the National Health Service over a follow-up period of about 8 years.

Sleep duration, sleep efficiency and mid-sleep were estimated using movement data, while cardiometabolic mortality was defined as any cause of death related to circulatory system disorders or endocrine and metabolic disorders. The study also found that disrupted circadian rhythm predicted a higher risk of mortality, which the authors were able to determine using computer modeling. The results took into account age, gender, ethnicity, photoperiod, and sociodemographic and lifestyle factors.

Research Into the Development of Therapeutic Lighting

According to the researchers, the results show how important it is to maintain a dark environment in the late night and early morning hours, when the central circadian “pacemaker” is most sensitive to light, and to seek out bright light during the day to improve our circadian rhythms. Lighting environment protection may be especially important for individuals at risk of both circadian disruption and increased mortality, such as those in intensive care units or nursing homes. In the general population, avoiding nighttime light and seeking daytime light may reduce the burden of disease, particularly cardiometabolic disease, and increase lifespan.

Previous research has reported the discovery of three types of cells in the eye that sense light and synchronize the brain’s circadian rhythm with our ambient light levels. These are photosensitive ganglion cells or (intrinsic) photosensitive ganglion cells (ipRGC), a type of neuron in the retina. Understanding how ipRGCs respond to the quality, quantity, duration and sequence of light will help us design better lighting for intensive care units, day care centers, schools, factories, offices, hospitals, senior living facilities and even the space station. Future research into the development of therapeutic lighting can also be advanced, with the aim of treating depression, insomnia, attention deficit hyperactivity disorder (ADHD), migraine pain and even sleep problems in patients with Alzheimer’s disease.