Australian, American, and British researchers conducted a prospective analysis of light exposure in nearly 89,000 people and concluded that greater light exposure at night and less during the day are associated with an increased risk of all-cause mortality. The study was published in the journal Proceedings of the National Academy of Sciences.
Constantly increasing nighttime light disrupts circadian rhythms, which are adapted to the natural cycle of day and night light, by shifting the timing (phase) and weakening the signal (amplitude) of their central regulator, the suprachiasmatic nuclei of the hypothalamus. This negatively impacts various aspects of physical and mental health, increasing the risk of many diseases, from retinal damage and stroke to depression, psychosis, and Alzheimer's disease. Animal experiments and population studies using satellite data have indicated that chronic disruptions of circadian rhythms due to artificial lighting can increase overall mortality, but the impact of personal light exposure in humans has not been studied previously.
To fill this gap, Daniel Windred of Flinders and Monash Universities and colleagues from Australia, the UK, and the US used data from 88,905 individuals (mean age 62.4 years; 57 percent women; 84.1 percent with an urban postcode) from the UK Biobank repository. Each participant wore a light sensor for a week, yielding nearly 13 million hours of data. Daytime and nighttime light exposure were determined using factor analysis of 24-hour light profiles. Computer modeling was used to determine their impact on the phase and amplitude of the brain's circadian pacemaker.
The mean follow-up period for participants was 7.99 years, with a total follow-up time of 9.55 years. During this time, 3,750 of them died (including 798 from cardiometabolic disorders); the all-cause mortality rate was 5.28 per 1,000 person-years. Death risks were calculated using Cox proportional hazards models adjusted for age, sex, ethnicity, daylight hours, sociodemographic factors, and lifestyle. Daytime and nighttime light exposure were categorized into intervals from 0 to 50, 50 to 70, 70 to 90, and 90 to 100 percentiles.
Higher nighttime light exposure was associated with an increased risk of all-cause mortality: hazard ratios (HRs) were 1.15–1.18 for the 70th–90th percentiles and 1.21–1.34 for the 90th–100th percentiles, compared with 0–50th percentiles. In contrast, higher daytime light exposure was associated with a decreased risk: HRs were 0.84–0.90 for the 50th–70th percentiles; 0.74–0.84 for the 70th–90th percentiles; and 0.66–0.83 for the 90th–100th percentiles, compared with 0–50th percentiles. For cardiometabolic mortality, the effects of nighttime and daytime light exposure, as well as circadian amplitude, on risk were even more pronounced.
Thus, minimizing light exposure at night and maximizing it during the day, as well as maintaining regular light-dark cycles that enhance circadian rhythms, may contribute to improved cardiometabolic health and increased lifespan, the authors conclude.
Fluorescent and LED lamps are primarily used for night lighting. They are distinguished by the fact that their light contains a significant portion of the blue spectrum. You can read about the health impact this can have and the current evidence available in the article "Only Blue Light Sucks the Eyes."
