Circadian rhythms used to be just a curiosity or an inconvenience when flying across time zones. But we now know robust daily cycles in almost every function our bodies are essential to good health. When our circadian rhythms are disrupted, by the use of blue-rich LED lights at night, and insufficient blue-rich light during the day, the risk of sleep disorders, obesity, diabetes, heart disease, as well as breast cancer and other endocrine-sensitive cancers are substantially increased.
Lifespan has long been known to be shortened in fruit flies (drosophila) and rodents when their circadian rhythms are disrupted in repeated experiments on jet lag. But what about humans?
Now several large-scale studies, some involving over 80,000 people with an average age of 60, have shown that disrupting circadian rhythms dramatically reduces human lifespan. These studies recorded how much sunlight each person was exposed to daily, how much light was in their bedroom at night, and how regular was the timing of their sleep. Then, for more than seven years the researchers tracked how many people died in each group, and from what causes.
The effects on longevity were enormous:
- People who stayed indoors under dim levels of lighting and rarely ventured outside into bright sunlight died at a 30 percent greater rate than those exposed to the most daily sunlight. Even though there were more skin cancers, the lengthened lifespan of those exposed to more sunlight overwhelmed any increased skin cancer risk. The effect of staying indoors and avoiding sunlight was as large as the increased risk of early death in people who smoked. Other studies show that morning sunlight has the most benefit.
- People who slept with the lights on in their bedroom at night died at a 30 percent faster rate, as compared to those who slept in pitch darkness. And deaths from cardiovascular (heart attacks) and metabolic (diabetes) diseases increased by 40 percent.
And the third study recently published in Sleep showed the biggest effect.
- People who sleep at irregular times die much sooner than those sleeping on a regular nightly schedule. Those sleeping on a regular schedule had up to a 48 percent reduced risk of early death from all causes and up to a 57 percent lower risk of early death from cardiac and metabolic diseases. Indeed, the regularity of sleep had a much bigger effect than how many hours people slept.
If you follow all three healthy behaviors (maximize time outdoors, especially in the mornings, sleep in the dark, and follow a precisely regular sleep schedule) will you live forever? Of course not, because there is a lot of overlap between these three behaviors. But they point to the essentials of a healthy lifestyle.
The common theme is that all three behaviors lead to robust circadian rhythms tightly entrained to the natural 24-hour day-night cycle.
While you are waiting for the breakthroughs in circadian lighting expected this year, which provide blue-rich daytime indoor light and zero blue evening light, there is much you can do to improve your chances of living a long and healthy life.
References
Lindqvist PG et al. (2014) Avoidance of sun exposure is a risk factor for all-cause mortality: results from the MISS cohort. J Intern Med 2014; 276: 77–86.
Windred et al (2023). Light at night and modeled circadian disruption predict higher risk of mortality: A prospective study in> 88,000 participants. medRxiv, 2023-09.
Windred DP et al (2024) Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study. SLEEP 47: 1–11 https://doi.org/10.1093/sleep/zsad253