Let There Be Light — and Darkness — for Our Circadian Rhythms

This post is a review of The Inner Clock: Living In Sync With Our Circadian Rhythms. By Lynne Peeples. Riverhead Books. 368 pp. $30.

In Time, Love, Memory, published in 1999, Jonathan Weiner suggested that “Inventing a clock was probably one of the first acts of life”. But scientists, Lynne Peeples reminds us, were only recently able to identify the timekeepers inside human beings, animals and plants that track 24-hour cycles, seasonal and lunar patterns.

In The Inner Clock, Peeples, a science journalist, draws on the latest research, interviews with astronauts and athletes, and her own experience living inside a Cold War-era bunker for 10 days without clocks or daylight, to provide an immensely informative and fascinating account of the impact of circadian rhythms on sleep, appetite, body temperature, metabolism, mood and immune responses; the ways in which industrialization, urbanization and technology have “ticked off” the 24-hour long cycles, synced with periods of daylight and darkness, of our internal clocks; and how to get them back on track.

Source: Kranich17/pixabay

These days, Peeples demonstrates, Americans and Europeans spend, on average, 90% of the day inside homes, schools, workplaces and elder care facilities, using artificial light. Designed with visibility, safety and energy efficiency in mind, electric light, Peeples writes, “lacks many biologically important wavelengths.” Tall buildings, moreover, cast shadows on streets. Pollution obscures the sun. Jet lag, time zones established across the continental United States, and Daylight Savings Time throw off our circadian systems.

Meanwhile, streetlights, porchlights, vehicular headlights, reflections off concrete, asphalt and snow, shine through windows, turning night into day in cities, suburbs, and some rural areas. By one estimate, the night sky became about 10 percent brighter each year between 2011 and 2022. While caffeine, alcohol, after-dark dinners, midnight snacks, movies on Netflix and Amazon Prime, and the light emanating from laptops and smartphones in bedrooms extend the day and discourage our bodies from producing sleep-signaling melatonin. Homes with LEDs are even more sleep-deprived than those using incandescent lights.

Peeples acknowledges that many studies linking disruptions in circadian rhythms to declines in productivity, depression, obesity, diabetes, susceptibility to infectious diseases and other maladies are far from definitive. She makes a compelling case, however, that “even when proof of causation may be lacking,” the evidence is sufficiently strong “to take a precautionary approach.” Deeply concerned about “the epidemic spread of light pollution,” moreover, she supports legislation adopted in France, Slovenia and the Republic of Korea regulating when, where and how lights are used and inclusion of a “right to light” in the United Nations 2030 Agenda for Sustainable Development.

Most importantly, Peeples presents a slew of recommendations, many of them simple and relatively inexpensive, to support and sustain internal body clocks. She agrees that schools should start one hour later and suggests scheduling exams in the afternoon. Wherever possible, work schedules should accommodate circadian rhythm “chronotypes,” which vary a bit from individual to individual. Blackout curtains and an eye mask will help sleepers in the summer, when the sun rises before it is time to wake up. Melatonin supplements should be used, but only when necessary. Tunable LEDs can compensate for some circadian losses, especially in nursing homes. They might also supply circadian cues, which fetuses begin to acquire in the second semester, to premature babies, and generate a sufficient amount of the right kind of light for the late-night trips of grown-ups to the bathroom. And regular exercise can strengthen circadian rhythms in skeletal muscles.

The Inner Clock also takes a glimpse into the future. To produce healthier food, Peeples points out, agricultural and horticultural scientists “are attempting to harness and hack the clocks in plants” to reduce, for example, the amount of water they need and prolong the shelf life of lettuce.

“We’ve ignored circadian rhythms for too long, to our peril,” Peeples concludes. “It’s time to reset and recover our inner clocks.”