第 1 章
The Timekeepers Within: How Our Bodies Track Every Moment
In December 2020, just before the Coast Guard icebreaker Polar Star departed Seattle for the Arctic's perpetual darkness, I sent its commanding officer a special lamp designed to mimic the sun's bright bluish-white light. This wasn't merely a thoughtful gift-it was a crucial tool for maintaining the crew's biological rhythms in an environment devoid of natural light cues. Within 72 hours of using the lamp in the mornings and filtering blue light in the evenings, Captain William Woityra reported dramatic improvements in sleep, mood, and eating patterns.
What Captain Woityra experienced firsthand was the profound influence of our internal timekeepers-miniature clocks ticking throughout our bodies, orchestrating everything from hunger and alertness to immune function and hormone release. This biological timekeeping system, which evolved to synchronize with the sun's predictable patterns, now faces unprecedented disruption in our modern world of artificial light, irregular schedules, and digital devices. The consequences of this misalignment extend far beyond occasional sleeplessness, potentially contributing to obesity, heart disease, depression, and countless other ailments that plague contemporary society.
第 2 章
The Hidden Orchestra: How Our Bodies Keep Time
Just before the Coast Guard icebreaker Polar Star departed Seattle in December 2020, I sent its commanding officer Captain William Woityra a special table lamp designed to emulate the sun's bright bluish white light-a key cue for our body's timekeeping. The COVID-19 pandemic had redirected their mission from Antarctica's endless daylight to the Arctic's perpetual darkness, and I worried about the crew's biological rhythms without proper sunlight.
At this very moment, miniature timepieces tick throughout your body-in your stomach, skin, liver, lungs, even bones and muscles-directing when you feel hungry, sleepy, alert, or strong. This circadian rhythm, orchestrated by a master timekeeper in your brain, evolved to sync with the sun's predictable patterns. But modern life disrupts these essential rhythms: artificial light, jet lag, contrived time zones, air pollution, and late-night meals leave our internal clocks beating to different drums, potentially causing obesity, heart disease, depression, and countless other ailments.
Living organisms have been tracking time long before human-made clocks existed. Evolution ingrained timekeeping tools that allow species to anticipate environmental changes rather than merely respond to them. This biological timing is crucial for survival activities like finding food, avoiding predators, and reproducing. The little penguins of Phillip Island, Australia demonstrate this precision by speed-waddling from ocean to burrow at precisely the same sun time each day. The parasite causing sleeping sickness leverages its time-tracking mechanism to disrupt host circadian rhythms. Honeybees perform intricate waggle dances that account for the sun's changing position to direct hivemates to food sources. For humans, our clock network regulates hormones, blood pressure, heart rate, and muscle function-with strength generally peaking around dusk when our ancestors would have returned from hunting. Our circadian rhythms govern sleep, mood, appetite, immune response and body temperature, all synchronized with Earth's 24-hour rotation through environmental cues like light and darkness.
Scientists hunting for the physical location of the circadian clock first explored cockroach and rat brains, discovering that severing certain neural connections disrupted their rhythms. They eventually identified the suprachiasmatic nucleus (SCN)-twin pinhead-sized clusters of about 20,000 neurons in the hypothalamus-as the master timekeeper in mammals, including humans. When transplanted between animals, the donor's rhythm transferred with the SCN. Meanwhile, molecular biologists led by Konopka and Benzer discovered the first clock gene (period) in fruit flies in 1971, revealing that genes control timekeeping. Further research uncovered complex feedback loops of proteins that build up and break down in roughly 24-hour cycles across species. These discoveries culminated in the 2017 Nobel Prize awarded to Michael Young, Jeffrey Hall, and Michael Rosbash for revealing the molecular machinery of circadian rhythms.
第 3 章
Time Blindness: The Bunker Experiment
I descended into the cool dimness of an Arkansas bunker, a decommissioned nuclear missile complex converted into a vacation rental. With each step down the concrete stairwell, daylight diminished to a tiny rectangle above before I faced a foot-thick steel blue door weighing "about the same as my truck." Beyond lay tunnels leading to a former Titan II missile silo and launch control center-my windowless home for the next ten nights.
I'd sought this underground sanctuary to understand my body's internal rhythms by avoiding all time cues-sunlight, clocks, people. Despite friends' concerns about my sanity, my host GT Hill, an Air Force veteran who spent over $500,000 remodeling the 3,500-square-foot facility, promised to help. We meticulously covered every digital clock with black electrical tape and devised communication strategies to ensure he never revealed the time.
I equipped myself with wearables to measure heart rate, activity, sleep patterns, light exposure, glucose, and body temperature. Without the sun's calibrating influence, my internal timekeepers would continue ticking but likely drift from precisely 24 hours. More concerning, my body's various clocks might desynchronize from one another, creating the unpleasant effects familiar from jet lag-headaches, digestive troubles, concentration problems.
By my third morning underground, I discovered I wasn't alone. Tiny fruit flies occasionally darted around my head while eating or reading. Rather than using the camouflage-printed flyswatters hanging in the kitchen, I befriended one fly, naming it Per-after the period gene discovered in fruit flies exactly fifty years earlier.
In September 1971, scientists Seymour Benzer and Ronald Konopka at Caltech made a breakthrough in circadian science by identifying three strains of mutant fruit flies with warped time perception-one with no rhythmicity, another with shortened 19-hour days, and a third with lengthened 28-hour days. All three mutations mapped to the same gene, which they named period or per for short. This discovery led to further breakthroughs in understanding the molecular mechanisms controlling circadian rhythms, eventually earning a Nobel Prize for researchers who built on this work. As Jonathan Weiner wrote in Time, Love, Memory, "Inventing a clock was probably one of the first acts of life."
Midway through my underground stay, my body's internal timekeeping went haywire. As Smarr had predicted, various rhythms began falling out of sync after my fifth sleep. My voice recordings document a "distant and grumbly" stomach, "sunken feeling," "heavy chest," and feeling "loopy," "woozy," and "weak." I experienced temperature dysregulation and unprecedented clumsiness, dropping not just juggling balls but also my voice recorder, Kindle, and harmonica. The symptoms, though more extreme, reminded me of jet lag from a recent Vietnam trip.
By my eighth night underground, my body had completely reversed its rhythm. I made blueberry pancakes thinking it was dinner time (guessing 5:00 p.m.), when it was actually breakfast time-just after 6:00 a.m. I had unwittingly shifted to working night shifts, just as the Cold War-era crew members had done in this same bunker decades earlier.
Weeks after my experiment, circadian experts analyzed my data. Larry Smarr called my results "textbook" and "beautiful"-showing clear circadian drift followed by recovery upon emergence. Ethan Buhr from the University of Washington noted how my initially strong rhythms descended into "mayhem" after four or five days.
My data revealed the progressive deterioration of my circadian system. The initially distinct peaks and valleys in my temperature patterns gradually flattened and drifted later each day. By day eight, my highs could barely be distinguished from my lows-the opposite of a robust rhythm. Even after returning to Seattle, my temperature rhythms remained weak and muddled for about a week.
More concerning was how my bodily systems desynchronized from each other. While all my rhythms (temperature, heart rate, glucose, sleep-wake cycle) ran longer than 24 hours, they did so at different rates. Buhr compared this to an orchestra where musicians can't see each other or the conductor-each player capable individually but collectively producing "garbage" when uncoordinated. This internal chaos explained why I physically felt like garbage.
第 4 章
The Sunflower Symphony: Nature's Timekeepers
Plant names sometimes sound silly or ill-fitting, but the sunflower truly earns its cheery title with enormous yellow petals resembling sunshine rays and its famous sun-following behavior. To observe this phenomenon firsthand, I visited Lora Lee Wicks' sunflower farm in Duvall, Washington, where hundreds of thousands of sunflowers were preparing to bloom. Walking through her property past chickens, rescued ducklings, and various flowers, we reached a massive field of green sunflower plants standing three to four feet tall. Nearly all had their backs to us, with budding flower heads pointing westward toward the setting sun.
Despite camping on uneven ground with minimal barriers against nature's distractions, I slept remarkably well under the stars. This connection to our natural environment may be fundamental to human health, as Micaela Martinez suggested-we are deeply enmeshed with natural rhythms. Traditional Chinese medicine has long emphasized this harmony between body, nature and cosmos, with vital energy (qi) circulating through different organs at specific times of day. Ancient Greeks recognized two concepts of time: chronos (measurable passage) and kairos (the opportune moment). The earliest documented observation of circadian rhythms came from Androsthenes of Thasos in the fourth century BCE, who noticed tamarind tree leaves closing at night and unfurling at sunrise.
Awakened by disturbing animal cries from a nearby slaughterhouse before my alarm, I ventured back into the sunflower field with my red-lensed headlamp. The plants had completely reoriented themselves during the night-now facing eastward toward the faint glow of the Milky Way on the horizon. Their leaves had also adjusted, with front leaves bowing down and back leaves perking up so their tops faced east. This nocturnal movement suggested the plants weren't simply following the sun but anticipating its return. Plant biologist Stacey Harmer's research confirmed this internal clock mechanism. Her team's experiments showed that even when researchers rotated potted sunflowers 180 degrees after sunset or placed them under stationary artificial light, the plants continued their rhythmic east-west movements, demonstrating an innate timekeeping ability independent of direct solar cues.
Harmer's sunflower experiments revealed that an internal clock mechanism drives their daily movements. Rather than passively following the sun, sunflowers anticipate its path, providing evolutionary advantages: flowers that face east at sunrise heat up faster, attracting more pollinators, displaying ultraviolet markings visible to bees, and producing larger seeds. As sunflowers mature, their stems stiffen and permanently fix eastward, explaining why older plants no longer track the sunset. Circadian rhythms appear everywhere in nature-from birds' roosting patterns to mosquitoes' evening activity to plants growing fastest at night.
Circadian rhythms are so fundamental to life that scientists debate whether biological clocks evolved from a single primordial timekeeper or were independently invented multiple times. Carrie Partch, who helped research cyanobacteria's clock mechanisms, notes that "everything has a clock, even bread mold," with plant clocks remaining particularly mysterious despite being the first studied. Scientists now understand that virtually all living organisms maintain internal timekeeping systems, with implications across all kingdoms of life. As with fruit fly and rodent research, findings in one species often apply broadly across the biological spectrum, revealing the universal importance of these ancient timing mechanisms that regulate gene expression and physiological functions.
第 5 章
Chronotypes: The Science of Personal Timing
Jeff Wilcox, who falls asleep at dinner tables, meetings, and even horror movies, discovered he has familial advanced sleep phase (FASP)-a condition affecting roughly one in three hundred people. Unlike most with sleep disorders, Wilcox embraced his extreme lark tendencies, leveraging early mornings for productivity throughout his career. His marriage to a night owl creates humorous domestic arrangements where they barely need a full-sized bed. Through studying people like Wilcox and Betsy Thomas (who sleeps from 5pm to 2am), researchers identified mutations in clock-regulating genes like per2 that speed up internal timekeeping, creating shortened circadian cycles of about 23.3 hours instead of the typical 24. These discoveries are helping scientists understand connections between circadian rhythms and various health conditions.
Sleep is universal across species, though animals sleep in remarkably different ways-sharks with eyes open, birds while flying, and penguins in microsleeps totaling eleven hours daily. Sleep requirements vary dramatically, from two hours for elephants to twenty for bats. For humans, both quantity (7-9 hours) and timing matter critically. Two intertwined but independent processes control our sleep: the homeostatic system (building sleep pressure through adenosine accumulation) and the circadian rhythm. When these systems conflict-like trying to sleep after an all-nighter when your circadian clock signals wakefulness-quality rest becomes nearly impossible. Americans now average an hour less sleep than their grandparents did in the 1940s, a concerning trend for long-term health.
Every person possesses a unique circadian signature that determines their chronotype-their natural tendency toward earlier or later hours. Beyond the familiar "morning larks" and "night owls," chronobiologist Till Roenneberg added "doves" to describe the roughly 75% of people who fall between extremes. Chronotypes can be measured through melatonin onset testing (DLMO) or questionnaires that identify your sleep midpoint. The distribution forms a bell curve with slight rightward skew, with fewer extreme early types than late types. This diversity likely evolved for survival, as evidenced by the Hadza tribe of Tanzania, who are all simultaneously asleep for only 18 minutes daily. Your chronotype has strong biological foundations-potentially including Neanderthal gene variants that speed up circadian rhythms-but modern life has artificially stretched the chronotype distribution, pushing most people toward later timing than our ancestors experienced.
About 80% of students and workers rely on alarm clocks to wake earlier than their bodies naturally would. This misalignment between biological and social time creates what Roenneberg calls "social jet lag"-the difference between sleep midpoints on workdays versus free days. Nearly 70% of people in industrialized countries experience at least an hour of this chronic jet lag, with a third suffering more than two hours. It's equivalent to flying west every Friday and east every Monday. The consequences are severe: increased risks of obesity, smoking, substance use, anxiety, depression, cardiovascular disease, metabolic disorders, and poorer cognitive and academic performance. Night owls suffer most severely due to conventional social schedules that conflict with their natural rhythms.
Our bodies function differently throughout the day as cellular clocks regulate energy distribution across various biological processes. The sleep homeostat and circadian rhythm work together to maintain alertness, with predictable patterns of energy peaks and dips. Most people experience peak alertness a couple hours after waking, followed by the notorious post-lunch dip when the circadian system temporarily fails to counter rising sleep pressure. These fluctuations mean there are optimal times for different activities-from taking tests to performing surgery to exercising-with our deepest circadian trough occurring between 2:00-5:00 a.m., when accidents and disasters often occur. Understanding these patterns allows us to schedule activities according to our personal rhythms, potentially improving safety, productivity, and performance.
第 6 章
Light: The Master Regulator of Our Internal Clocks
Earth's formation established the fundamental rhythms that would shape all life. Initially rotating every six hours, Earth eventually settled into its 24-hour cycle while maintaining its annual orbit around the sun. A collision with a Mars-sized planet created our moon and tilted Earth's axis, generating seasons and varying day lengths across hemispheres. Life evolved under these inescapable daily, lunar, seasonal and annual cycles, developing internal clocks to anticipate environmental changes rather than merely react to them. These biological clocks aren't perfect timepieces-they naturally drift and require calibration through zeitgebers ("time givers"), with sunlight being the most powerful. Scientists only recently discovered that beyond rods and cones for vision, our eyes contain intrinsically photosensitive retinal ganglion cells (ipRGCs) that transmit light information directly to the suprachiasmatic nucleus, allowing our master clock to conduct the orchestra of cellular timepieces throughout the body.
The discovery of circadian photoreceptors began with Clyde Keeler, a Harvard graduate student and "mouse fancier" in the 1920s. Despite his blind mice lacking functional rods and cones due to retinal degeneration, their pupils still constricted when exposed to light. This finding suggested unknown light-sensitive cells must exist in the retina, though his discovery was largely ignored for sixty years. Russell Foster later replicated and extended Keeler's work, showing visually blind mice maintained normal circadian responses to light. Despite scientific skepticism, Foster engineered mice without rods or cones that still synchronized to light-dark cycles. Scientists eventually discovered that some blind people without rods and cones could subconsciously sense light and maintain circadian rhythms, confirming the existence of a third photoreceptor pathway.
Derek Naysmith lost both eyes in a fireworks accident in 1986 at age thirty-three. Without eyes, he lost not only rods and cones but also ipRGCs (intrinsically photosensitive retinal ganglion cells). His SCN no longer received light cues, causing his internal clock to run on its own nearly 25-hour cycle. He developed non-twenty-four-hour sleep-wake rhythm disorder, falling increasingly out of sync with his family and struggling at work. In the mid-1990s, he joined a sleep study led by Steve Lockley, who introduced him to synthetic melatonin treatment as a replacement time cue. With this intervention, Naysmith found that "life became livable again."
On a cloudy Seattle day, Russell Van Gelder explained that despite the gray appearance, the sky still contained the full range of wavelengths comparable to a sunny day-just fewer photons. Light intensity is measured in lux: bright sunlight reaches 100,000 lux, cloudy days between 1,000-10,000 lux, and indoor environments typically 25-250 lux. While our visual system (rods and cones) evolved to rapidly adjust to extreme contrasts, melanopsin in our third photoreceptors "plays the long game"-responding sluggishly but persistently to light, requiring thousands of times more intensity than visual photoreceptors need. Melanopsin prefers shorter blue wavelengths around 480 nanometers, which are abundant outdoors even on cloudy days but limited in indoor lighting, creating a significant mismatch for our circadian system.
Beyond regulating our circadian rhythms, light directly affects alertness, mood, and cognition through pathways independent of the SCN. These "masking" effects can override circadian expression-as demonstrated during solar eclipses when nocturnal animals suddenly become active while diurnal ones prepare for sleep. Samer Hattar theorizes that a separate light pathway works alongside the circadian system and sleep homeostat, suggesting evolution provided multiple, sometimes redundant strategies for detecting environmental cues.
第 7 章
The Inequality of Light and Dark
Walking London's streets with Andreas Billman, I discovered "blind windows"-bricked-up facades created during England's 1696-1851 window tax era. The tax, designed to assess wealth by counting windows, led property owners to block windows rather than pay, leaving poor families in darkness. Architects adapted by designing buildings with fewer windows. Similar taxes emerged across Europe, with France taxing both quantity and size of windows until 1926. Though England eventually lifted the tax, inequitable "daylight robberies" continue today through artificial lighting.
Following the Industrial Revolution and window tax repeal, architects initially built narrow structures with abundant windows. This changed with Edison's light bulb, leading to deeper buildings reliant on electric illumination. Today, average Americans spend 90% of their time indoors, often without windows, experiencing a thousandfold drop in light exposure lacking biologically important wavelengths. The Old Order Amish, who avoid grid electricity, maintain natural day-night connections, experiencing tenfold greater light contrast between day and night than the general population. Walking through Seattle with architect Marty Brennan revealed stark differences: 71,584 lux by water, but just 138 lux inside a market-bright enough to see by but insufficient for proper circadian function. "We are experiencing biological darkness in our indoor environments," Brennan explained.
The "ancient lights" signs I spotted throughout London represent an English property law from 1663 (updated in 1832) that protects homeowners' right to natural light through windows they've enjoyed for twenty years. This concept dates back to Roman emperor Justinian and ancient Greece. Similar daylight protection regulations exist worldwide-Bali limits buildings to fifteen meters, Chinese cities mandate minimum sunshine hours, and Japanese nisshoken governs building heights to prevent excessive shadowing. Many European cities maintain wide streets and lower buildings to maximize daylight access, though economic pressures have led some cities to relax height restrictions despite the consequences for public light access.
America rejected the "ancient lights" doctrine in 1838, prioritizing economic expansion over daylight rights. Though some cities eventually adopted building height restrictions and setback requirements-like those giving the Empire State Building its wedding cake shape-problems persist. Manhattan neighborhoods now spend half the day in shadow, with intersections like Cedar and William among the darkest in New York. Inside apartments, residents encounter what the British call the "grumble line"-the threshold beyond which light becomes inadequate for reading. As skyscrapers like Central Park Tower rise along "Billionaires' Row," the wealthy enjoy unobstructed light in multi-million dollar penthouses while casting shadows on more affordable homes and public parks below, perpetuating a daylight inequality that has historically burdened the urban poor since the tenement era documented by Jacob Riis.
Modern energy-efficient windows inadvertently filter out crucial light wavelengths. Daylight expert Lisa Heschong noticed plants thriving near old windows but suffering near new ones because low-emissivity (low-e) coatings, mandated by building codes for energy efficiency, increasingly restrict light to only visible wavelengths around 555 nanometers. These coatings block both ultraviolet and infrared light, potentially undermining health benefits from both short-wavelength blue-violet photons and long-wavelength red and near-infrared light. The newest triple silver low-e coatings can block 70-85% of circadian-stimulating light, compounding our indoor light deficiency problem.
第 8 章
The Darkness Epidemic: Light Pollution and Health
Viewing Earth from space reveals a planet ablaze with artificial light, with coastal cities and transportation corridors forming luminous patterns visible even without political boundaries. Over 99 percent of Americans and Europeans now live under light-polluted skies, where streetlights, headlights, billboards and even natural gas flaring in rural areas have transformed night into a perpetual twilight. The narrator describes planning a trip with his father to central Oregon, seeking one of the few remaining truly dark places to reconnect with the stars they once enjoyed together.
For billions of years, life evolved under natural darkness, with only moonlight, lightning, and wildfires as light sources. Today's artificial lighting has profound biological consequences beyond obscuring the stars. Indoor and outdoor lighting disrupts our circadian rhythms by preventing melatonin production at sundown. Studies show homes with enough light to halve melatonin secretion, with LED lighting causing worse disruption than incandescent bulbs. This circadian disruption has been linked to depression, obesity, poor blood sugar control, reduced fertility, preterm births, and even increased COVID-19 severity. Even low light levels equivalent to a hallway light or television can raise sleeping heart rates and morning insulin levels.
Artificial light devastates wildlife across ecosystems. Birds sing prematurely and migrate off course, while even eyeless creatures like oysters suffer circadian disruption from light pollution below full moon intensity. Most alarming is light's role in global insect collapse-moths fly to their deaths, dung beetles lose their Milky Way navigation, and fireflies can't communicate through bioluminescence. These impacts ripple through ecosystems, threatening pollination and food security. Research shows insect visits to plants dropped by nearly two-thirds under artificial light. Light pollution also extends mosquito seasons, delays plant dormancy periods, and disrupts marine carbon sequestration. As UCLA's Travis Longcore argues, addressing light pollution benefits both wildlife and human health simultaneously.
Constant illumination, recognized as torture in Orwell's 1984 and deemed unconstitutional for prisoners, has become normalized in disadvantaged communities. Light pollution is fundamentally a class issue-in Baltimore, just two miles separate harshly lit low-income neighborhoods from warmly illuminated wealthy areas. Research confirms people of color and low-income residents face twice as much nighttime light exposure as white Americans, with poorer sleep quality directly linked to higher light levels. Disadvantaged families face compounding hardships: homes closer to streetlights, inability to afford blackout blinds, and shared sleeping spaces. In both London and New York City, I witnessed the stark contrast between gentle yellow lighting in wealthy neighborhoods versus prison-yard-like LED illumination in social housing complexes. While some residents appreciate nighttime visibility for safety and recreation, many find the constant glare excessive and disruptive.
第 9 章
Resetting Our Rhythms: Solutions for Modern Life
In 1931, Mary Smith earned sixpence weekly as a "knocker up," shooting dried peas at windows to wake factory workers. Before alarm clocks became affordable, these human alarm clocks were essential in the Industrial Revolution era when workers needed to rise by the clock rather than the sun. The Smith family, like others in this "economy of waking," charged more for earlier wake-ups and winter service. Despite initial resistance to mechanical alarm clocks (one critic called them "devil screamers"), they eventually replaced knocker ups, and now smartphones have nearly rendered traditional alarms obsolete. Yet circadian scientists hope we might one day abandon all alarm clocks. Waking naturally with our internal timers would be healthier, though impractical for many in our current work structure.
After ten alarm-free nights in the bunker experiment, I've tried avoiding alarms whenever possible-a luxury my flexible profession allows. Most Americans naturally wake between 8-9am after midnight bedtimes, making typical 8-9am work starts problematic. Despite most people being owls or doves, society favors larks with proverbs like "The early bird gets the worm." Camilla Kring, founder of Copenhagen's B-Society, advocates for "B-persons" (night owls) to have the right to live in sync with their biological clocks. She's helped companies like Medtronic and AbbVie implement chronotype-matched schedules, with meetings held midday. At AbbVie Norway, this flexibility increased work-life balance satisfaction from 58% to 95%, reduced turnover and sick leave, and earned them "best place to work" awards. Employees discovered their peak productivity hours-like HR director Marte Fjelle, who worked best between 3-6pm, making her "double as effective" as at 9am.
Magne Skram Hegerberg, general secretary for the Norwegian Association of Lawyers, ceremoniously removed the office clock-in machine with a mock funeral complete with candles. His forty-plus employees now start between 6:30am and 2:30pm based on their chronotypes, which has doubled productivity in some areas and improved innovation and problem-solving. Chronobiologist Till Roenneberg endorses this approach: "If I want to have a perfect workforce, I would tell them to come when they have woken up without an alarm." His research showed significant improvements in sleep and reduced social jet lag among factory workers on chronotype-adjusted schedules. Kring provides companies with plush frogs that employees place on desks during their power hours to signal "frog off!"-a request not to be interrupted. She advocates for chronotype discrimination to be added to EU and UN human rights declarations, and has extended her work to nursing homes like GuldBoSund, where residents choose their own wake-up times rather than following staff convenience. Eighty-eight-year-old resident Bent, a former plumbing company owner, now enjoys waking at 9am: "I'm going to bed when it fits me, not when someone tells me to." Staff sick days have also decreased significantly, even among night shift workers.
In Malmo, Sweden, I discovered how artificial lighting is being engineered to support human circadian rhythms. The Malmo Redhawks hockey team's windowless locker room features tunable LED fixtures that mimic natural sunlight patterns, with bright bluish light before games and training, and softer orangish light afterward. The system adjusts intensity and spectrum 14,000 times daily to approximate an April day in Sweden, helping players maintain consistent circadian rhythms despite spending long hours indoors during short winter days.
My journey into circadian lighting began unexpectedly in 2014 with the Seattle Mariners baseball team, early adopters of preset LEDs for pregame stimulation and postgame relaxation. Since then, numerous professional sports teams have embraced this technology seeking performance advantages. But sports represent just a fraction of the growing circadian lighting market, which now targets schools, offices, hospitals, nursing homes, and windowless workplaces like call centers and warehouses.
The convergence of LED innovation, energy efficiency demands, and advancing scientific understanding of light's biological impacts has sparked a lighting revolution. Tunable LED systems can theoretically restore the natural light-dark contrast missing from our indoor lives by adjusting color, intensity and timing-potentially keeping our clocks in sync, tracking seasonal changes, improving alertness, boosting memory and mood, and preventing circadian-related health problems. These programmable systems can deliver specific wavelengths at appropriate times, moving beyond simple on-off functionality. Though circadian lighting systems have existed since around 2013 (with the International Space Station being an early high-profile installation in 2016), higher costs and energy efficiency concerns have slowed widespread adoption.
