Australian, American and British researchers conducted a prospective analysis of light levels in almost 89,000 people and concluded that greater exposure to light at night and less during the day were associated with an increased risk of death from all causes. A report on the work 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 alternation of day and night light, by shifting the time (phase) and weakening the signal (amplitude) of their central regulator, the suprachiasmatic nuclei of the hypothalamus. This negatively affects various aspects of physical and mental health, increasing the risk of many diseases - from retinal damage and stroke to depression with psychosis and Alzheimer's disease. Animal experiments and population studies using satellite data have indicated that chronic disruption of circadian rhythms due to artificial lighting can increase overall mortality, but the impact of personal exposure to light in humans has not been studied before.
To fill this gap, Daniel Windred of Flinders and Monash Universities and colleagues in Australia, the UK and the US used data from 88,905 people (average age 62.4 years; 57 per cent women; 84.1 per cent with an urban postcode) in the UK Biobank repository. Each wore a light sensor for a week, totalling almost 13 million hours of data. Daytime and nighttime light exposure were determined by factor analysis of 24-hour light profiles, and computer modelling was used to determine their effects on the phase and amplitude of the brain’s circadian pacemaker.
The mean follow-up of participants was 7.99 years, with a total follow-up of 9.55 years. During this time, 3750 of them died (including 798 from cardiometabolic disorders); all-cause mortality was 5.28 per 1000 person-years. Risks of death were calculated using Cox proportional hazards models adjusted for age, sex, ethnicity, daylength, sociodemographic factors, and lifestyle. Daytime and nighttime light exposure were binned as 0 to 50, 50 to 70, 70 to 90, and 90 to 100 percentiles.
Higher light exposure at night was associated with an increased risk of all-cause mortality: hazard ratios (HRs) of 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 of 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 and circadian amplitude on risk were even stronger.
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 mainly used for night lighting. They are distinguished by the fact that their light contains a significant portion of the blue part of the spectrum. You can read about the impact this can have on health and what confirmed data is currently available in the article “Only Blue Sucks Your Eyes.”
