Jet Lag Calculator

A jet lag calculator is a specialized chronobiological scheduling algorithm designed to compute the exact times a transmeridian traveler should seek light, avoid light, and sleep in order to rapidly align their internal circadian pacemaker with a new time zone. By mapping the mathematical phase response curve of the human biological clock, these calculators solve the physiological desynchronization that causes the exhaustion, cognitive impairment, and gastrointestinal distress collectively known as jet lag. Readers of this comprehensive reference will master the biological mechanics of circadian shifting, the specific mathematical formulas used to calculate adaptation schedules, and the expert protocols required to manipulate environmental cues and conquer travel fatigue completely.

What It Is and Why It Matters

A jet lag calculator is a deterministic, math-based tool that translates the complex biology of the human circadian rhythm into a precise, actionable daily schedule for travelers crossing multiple time zones. When a human being travels rapidly across longitudes, their internal master clock—located in the brain—remains synchronized to the light-dark cycle of their departure city, while their physical body is subjected to the environmental cues of the destination. This temporal mismatch is called desynchronosis, and it results in severe physiological consequences, including a documented 30% reduction in cognitive processing speed, profound sleep maintenance insomnia, daytime hypersomnolence, and suppressed immune function. A jet lag calculator solves this problem by utilizing optimal control theory to calculate the fastest possible biological pathway to realign the internal clock with the new external environment. It tells the user precisely when to expose their eyes to bright light, when to block all light, when to ingest synthetic melatonin, and when to sleep.

The necessity of this calculation stems from the counterintuitive nature of biological timekeeping. Simply adopting the local schedule upon arrival—the most common strategy employed by travelers—frequently exacerbates jet lag by exposing the traveler to light at the exact wrong biological moment, thereby shifting their internal clock in the opposite direction of the destination. For multinational corporations, the stakes are measured in billions of dollars of lost productivity and botched negotiations due to cognitively impaired executives. For elite athletes, a circadian misalignment of even two hours can reduce peak muscular output by 1.5% to 3%, which is the difference between an Olympic gold medal and failing to qualify. By relying on a jet lag calculator, travelers replace guesswork with hard biological science, minimizing the duration of their desynchronosis from weeks down to a matter of days.

History and Origin

The conceptual foundation for the jet lag calculator began in the late 1950s, coinciding directly with the dawn of the commercial jet age. In October 1958, Pan American World Airways inaugurated the first daily transatlantic jet service using the Boeing 707, suddenly allowing humans to cross five time zones in eight hours—a speed of travel that outpaced the human body's evolutionary capacity to adapt. The following year, in 1959, Romanian-American scientist Franz Halberg coined the term "circadian" (from the Latin circa meaning "around" and diem meaning "day") to describe the endogenous 24-hour biological cycles he observed in organisms. By 1966, the specific physiological distress caused by rapid transmeridian flight was officially named "jet lag" by Hubertus Strughold, a pioneer in aerospace medicine. However, for decades, the only advice given to travelers was to "tough it out" or rely on heavy sedative-hypnotic drugs to force unconsciousness.

The breakthrough that made modern jet lag calculators mathematically possible occurred in 1989, when Dr. Charles Czeisler at Harvard Medical School published a landmark study detailing the exact Phase Response Curve (PRC) to light in humans. Czeisler proved that exposure to bright light could shift the human biological clock by up to 12 hours, but crucially, the direction of the shift (earlier or later) depended entirely on the exact core body temperature of the subject at the time of light exposure. This discovery proved that circadian shifting was a solvable mathematical equation. The transition from laboratory science to consumer calculators occurred in the 2010s, culminating in 2014 when mathematicians Daniel Forger and Olivia Walch at the University of Michigan released the "Entrain" algorithm. By applying optimal control theory—the same mathematics used to calculate spacecraft trajectories—they created a mobile computational model that output the mathematically fastest schedule of light exposure to overcome jet lag, establishing the exact framework used by all professional jet lag calculators today.

Key Concepts and Terminology

To understand how a jet lag calculator functions, one must first understand the specific physiological and mathematical vocabulary it relies upon. The Circadian Rhythm is the roughly 24.2-hour endogenous cycle of physiological processes operating in all human beings, dictating everything from hormone secretion to cellular repair. The master pacemaker controlling this rhythm is the Suprachiasmatic Nucleus (SCN), a cluster of 20,000 neurons located in the hypothalamus directly above the optic chiasm. The SCN is entirely blind; it relies on Zeitgebers (German for "time-givers"), which are external environmental cues that entrain or synchronize the internal clock to the 24-hour solar day. The single most powerful zeitgeber is high-intensity, short-wavelength light (specifically blue light at the 480-nanometer wavelength), followed secondarily by food intake and physical exercise.

A jet lag calculator measures the state of the SCN by estimating the user's Core Body Temperature Minimum (CBTmin). The CBTmin is the exact moment during the 24-hour cycle when the body's internal temperature drops to its absolute lowest point, typically occurring 2 to 3 hours prior to a person's natural wake time. The CBTmin acts as the mathematical anchor point for all jet lag calculations because it represents the inflection point of the Phase Response Curve (PRC). The PRC is a graph showing how the timing of a zeitgeber affects the circadian clock. If light hits the retina in the hours immediately before the CBTmin, it causes a Phase Delay, shifting the biological clock later. If light hits the retina in the hours immediately after the CBTmin, it causes a Phase Advance, shifting the clock earlier. A jet lag calculator's primary function is to map the user's CBTmin and schedule light exposure precisely on the correct side of this inflection point.

The Biology of Jet Lag

The biological mechanism that a jet lag calculator manipulates begins in the human eye, specifically within a specialized class of cells discovered in 2002 called Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs). Unlike rods and cones, which form visual images, ipRGCs do not contribute to conscious sight. Instead, they contain a photopigment called melanopsin, which is highly reactive to the blue light present in natural sunlight. When sunlight enters the eye, the ipRGCs fire electrical signals down the retinohypothalamic tract directly into the Suprachiasmatic Nucleus (SCN). Upon receiving this signal, the SCN immediately sends an inhibitory message to the pineal gland, halting the production of melatonin, the hormone responsible for signaling the biological night. This intricate biological pathway is what locks human physiology to the local time zone.

When a traveler crosses multiple time zones, this pathway is thrown into chaos. If a traveler flies from New York to London and lands at 7:00 AM local time, the sun is shining brightly, and the ipRGCs signal the SCN that it is morning. However, the SCN's internal chemical state believes it is 2:00 AM. This conflict between the exogenous light signal and the endogenous chemical state causes the SCN to send fragmented, conflicting signals to the billions of peripheral clocks located in the liver, kidneys, heart, and muscle tissue. The result is biological anarchy: the stomach secretes digestive enzymes while the traveler is trying to sleep, and the brain secretes melatonin while the traveler is trying to conduct a business meeting. Because the natural, unassisted adaptation rate of the human body is only about 1 hour per day for eastward travel and 1.5 hours per day for westward travel, a traveler crossing six time zones will suffer this biological anarchy for nearly a week unless they use a calculator to artificially manipulate their light exposure.

How It Works — Step by Step

A jet lag calculator computes the optimal adaptation schedule through a deterministic, multi-step algorithmic process. First, the calculator defines the time zone differential ($\Delta TZ$) by subtracting the origin time zone offset from the destination time zone offset. Next, it determines the direction of travel: a positive $\Delta TZ$ indicates Eastward travel (requiring a Phase Advance), while a negative $\Delta TZ$ indicates Westward travel (requiring a Phase Delay). The calculator then asks for the user's normal sleep schedule in their home time zone to pinpoint their Core Body Temperature Minimum (CBTmin). The standard formula estimates CBTmin to occur 2.5 hours before the user's habitual wake time. Using the Phase Response Curve rules, the calculator then generates a schedule: to advance the clock (Eastward), the user must seek bright light for 4 hours immediately after their CBTmin and strictly avoid light for 4 hours before their CBTmin. To delay the clock (Westward), the user must seek bright light for 4 hours before their CBTmin and avoid light after it.

To illustrate this, consider a full worked example of a traveler flying from New York (UTC-5) to London (UTC+0). The time zone difference ($\Delta TZ$) is +5 hours, meaning the traveler must achieve a 5-hour Phase Advance. The traveler normally sleeps from 23:30 to 07:30 in New York.

1. Calculate Origin CBTmin: Habitual wake time (07:30) minus 2.5 hours equals a CBTmin of 05:00 New York time.
2. Translate to Destination Time: 05:00 New York time is 10:00 London time.
3. Apply the PRC Rule: To Phase Advance, the traveler needs light after 10:00 London time and must avoid light before 10:00 London time.
4. Generate Day 1 Schedule: The traveler lands in London at 07:00. The calculator instructs them to wear dark sunglasses and avoid all bright light from 07:00 to 10:00. At exactly 10:00 London time, they must remove the sunglasses and seek bright outdoor sunlight until 14:00.
5. Iterate for Day 2: This Day 1 protocol successfully advances the internal clock by 1 hour. On Day 2, the CBTmin shifts earlier to 09:00 London time. The calculator updates the schedule: avoid light until 09:00, then seek light from 09:00 to 13:00. This process mathematically repeats until the CBTmin aligns with 05:00 London time, achieving total circadian synchronization.

Types, Variations, and Methods

Jet lag calculators generally utilize three distinct methodological approaches depending on the traveler's goals, the duration of the trip, and the flexibility of their schedule. The first and most aggressive method is Pre-Adaptation Shifting. In this variation, the calculator generates a schedule that begins 3 to 4 days before the traveler even boards the airplane. The user gradually shifts their sleep, wake, and light exposure times by 60 to 90 minutes each day while still at home. By the time they step off the plane at their destination, their circadian rhythm is already perfectly aligned with the local time. This method is highly favored by elite athletes and performing artists who cannot afford a single day of compromised performance upon arrival, though it requires immense discipline and control over one's home lighting environment.

The second variation is Destination-Only Adaptation, which is the standard method used by most commercial calculators. This algorithm assumes the user cannot alter their schedule prior to the flight due to work or family obligations. The calculator begins the phase shifting protocol from the moment the user steps onto the aircraft or lands at the destination. While more convenient, this method means the user will experience 2 to 4 days of active jet lag upon arrival while the calculator works to shift their clock retrospectively. The third variation is the Short-Trip Compromise Method. If a user inputs a trip duration of less than 48 to 72 hours, advanced calculators will abandon the goal of circadian alignment entirely. Instead, they calculate a "compromise phase" or anchor schedule, advising the user on how to maintain their home time zone biologically while functioning socially in the new time zone, thereby preventing the severe "double jet lag" that occurs when returning home.

Real-World Examples and Applications

To understand the practical power of a jet lag calculator, examine the scenario of a 35-year-old software engineer traveling from San Francisco (UTC-8) to Tokyo (UTC+9) for a critical 10-day deployment. This represents a massive 17-hour time difference, which chronobiologically translates to a 7-hour Westward phase delay (since shifting 7 hours backward is biologically faster than shifting 17 hours forward). The engineer normally wakes at 07:00 in San Francisco, placing their CBTmin at 04:30 SF time, which is 21:30 (9:30 PM) in Tokyo. If the engineer simply tries to "adapt" by walking around Tokyo in the morning sunshine at 08:00, they are getting light 13 hours after their CBTmin, which falls in the dead zone of the Phase Response Curve, resulting in zero biological adaptation and ensuring a full week of debilitating fatigue. By using a calculator, the engineer is instructed to seek bright light late in the Tokyo evening (prior to 21:30) and take 0.5mg of melatonin at 10:00 AM Tokyo time, shifting their clock by 1.5 hours per day and achieving full synchronization in just 4.6 days.

Another distinct application is found in professional sports. Consider an NFL team traveling from Miami (UTC-5) to London (UTC+0) for an International Series game on Sunday. The 5-hour Eastward shift is notoriously difficult because the human circadian clock naturally runs slightly longer than 24 hours (averaging 24.2 hours), making it biologically much easier to delay the clock (stay up later) than to advance it (wake up earlier). The team's sports scientists use jet lag calculators to initiate a pre-adaptation protocol on Wednesday. The calculator dictates that players must be exposed to 10,000 lux light boxes at 06:00 Miami time, practice earlier in the day, and wear blue-light-blocking glasses starting at 18:00 Miami time. By enforcing this mathematically calculated light regimen, the players arrive in London on Friday with their core body temperatures already shifted by 3 hours, ensuring their peak muscular output and reaction times align perfectly with the 14:00 kickoff in London.

Common Mistakes and Misconceptions

The most pervasive and damaging misconception regarding jet lag is the belief that exposing oneself to bright local sunlight immediately upon arrival is always the best way to adapt. As the mathematics of the Phase Response Curve demonstrate, if a traveler flies East and lands in the morning, local morning sunlight will often strike their retinas before their Core Body Temperature Minimum has occurred. This causes a Phase Delay, pushing their internal clock westward, mathematically worsening their jet lag and prolonging their recovery by several days. A jet lag calculator explicitly prevents this error by calculating the exact hour when morning light transitions from being harmful (delaying) to helpful (advancing), frequently requiring travelers to wear dark sunglasses outdoors during their first few mornings in a new city.

Another critical mistake involves the misuse of melatonin. The general public often views melatonin as a traditional sleep-inducing sedative, purchasing high-dose pills ranging from 5mg to 10mg and taking them right before bed in the new time zone. Chronobiologists and jet lag calculators treat melatonin entirely differently: it is a phase-shifting hormone, not a sedative. Calculators recommend physiological doses of just 0.3mg to 0.5mg, taken at highly specific times calculated relative to the CBTmin, often hours before bedtime. Taking a massive 10mg dose floods the brain's receptors, causing a "spillover" effect that blurs the circadian signal, leaves the user groggy the next day, and completely fails to shift the biological clock. Furthermore, many travelers mistakenly rely on caffeine to "cure" jet lag. While caffeine successfully blocks adenosine receptors to mask sleep pressure, it has zero effect on the Suprachiasmatic Nucleus; it does not shift the biological clock a single minute, leaving the underlying desynchronosis completely unresolved.

Best Practices and Expert Strategies

Professionals who travel extensively rely on strict adherence to the calculator's outputs combined with environmental controls to ensure rapid adaptation. The most critical best practice is the militant management of light exposure. When a calculator dictates "light avoidance," experts do not simply stay indoors; they wear specialized blue-light-blocking glasses equipped with amber lenses that filter out 99% of wavelengths below 530 nanometers. This ensures that even if they are forced to be in a brightly lit airport terminal or office, their Suprachiasmatic Nucleus perceives biological darkness, allowing the phase shift to continue uninterrupted. Conversely, when the calculator dictates "light exposure," experts do not rely on dim hotel room lamps, which emit a negligible 200 to 300 lux. They seek direct outdoor sunlight (10,000 to 100,000 lux) or use portable, medical-grade circadian light boxes positioned 16 inches from their face to guarantee the biological signal is registered.

Another expert strategy integrated with jet lag calculation is the manipulation of peripheral clocks through fasting and feeding schedules. While light shifts the master clock in the brain, food intake shifts the peripheral clocks in the digestive system and liver. Experts will often combine the calculator's light schedule with the "Argonne Anti-Jet-Lag Diet" principles. They will fast for 12 to 16 hours during the flight, breaking the fast with a high-protein meal at the exact time of the first scheduled breakfast in the destination time zone. This dual-pronged approach—using the calculator for light to shift the central SCN, and fasting to shift the peripheral organs—accelerates the total adaptation rate by up to 20%, significantly reducing the gastrointestinal distress and bloating typically associated with long-haul travel.

Edge Cases, Limitations, and Pitfalls

While jet lag calculators are incredibly powerful, they possess distinct limitations and mathematical breaking points, the most prominent being the phenomenon of "anti-dromic shifting." When a traveler crosses 9 to 12 time zones—such as flying from New York to Beijing (a 12-hour difference)—the internal clock is situated exactly opposite to the external environment. In these extreme edge cases, the calculator must decide whether to advance the clock forward by 12 hours or delay it backward by 12 hours. Because the human clock naturally drifts later, calculators will almost always default to a massive Phase Delay (shifting Westward). However, if the user inadvertently gets a brief exposure to bright light at the wrong moment, the biological clock may "snap" in the opposite direction, attempting to advance rather than delay. This anti-dromic shift tears the circadian rhythm in two different directions, resulting in a state of prolonged biological confusion where adaptation halts entirely, leaving the traveler violently fatigued for over a week.

Another significant limitation involves extreme chronotypes—individuals who are severe "night owls" or extreme "morning larks." Jet lag calculators rely on the mathematical assumption that a user's Core Body Temperature Minimum occurs exactly 2.5 hours before their natural wake time. However, in extreme night owls, the CBTmin might occur just 1 hour before wake time, or even shortly after waking. If a user inputs standard sleep times without accounting for their severe chronotype, the calculator's estimated CBTmin will be incorrect. Consequently, the calculator might instruct the user to seek light at 07:00, believing it is after their CBTmin and will cause a Phase Advance. If the user is a severe night owl, 07:00 might actually be before their CBTmin, resulting in a Phase Delay. This pitfall highlights the necessity for users to input their genuine, unconstrained biological sleep times, not the artificial sleep times imposed upon them by their work schedules.

Industry Standards and Benchmarks

The foundational algorithms powering modern jet lag calculators are derived from stringent industry standards developed by aerospace and military organizations. The most prominent benchmark is the NASA Fatigue Countermeasures Program, developed in conjunction with the Ames Research Center to manage the circadian rhythms of astronauts and flight controllers. NASA's standard establishes that the maximum safe rate of circadian adaptation is 1 hour per day for phase advances (Eastward) and 1.5 hours per day for phase delays (Westward). Any calculator claiming to shift a user's biology faster than these biological speed limits is disregarding accepted scientific benchmarks. Furthermore, the Federal Aviation Administration (FAA) utilizes these exact chronobiological calculations in their Part 117 regulations, which dictate the mandatory rest periods for commercial airline pilots to prevent catastrophic fatigue-related errors.

In the realm of elite athletics, the United States Olympic & Paralympic Committee (USOPC) employs a benchmark standard known as the "1-for-1 rule" when planning travel logistics: athletes are required to arrive in the host country one full day early for every single time zone crossed, unless a strict chronobiological phase-shifting protocol (a jet lag calculator) is utilized. When utilizing a calculator, sports scientists aim for a benchmark of a 2-hour phase shift per day using a combination of 10,000-lux light therapy, 0.5mg of melatonin, and precisely timed exercise. These institutional standards validate the mathematical models used in consumer jet lag calculators, proving that they are not mere travel hacks, but rather medical-grade protocols utilized by organizations where human performance is a matter of life, death, or global competition.

Comparisons with Alternatives

When evaluating the efficacy of a jet lag calculator, it is essential to compare it against the traditional alternatives employed by travelers, the most common being sedative-hypnotic medications such as Zolpidem (Ambien) or Zopiclone. While these pharmaceuticals successfully force the brain into a state of unconsciousness, allowing the traveler to sleep on the airplane or in the new time zone, they are biologically inert regarding the Suprachiasmatic Nucleus. They do absolutely nothing to shift the underlying circadian clock. Consequently, a traveler using sleeping pills will wake up in the new time zone and still experience severe daytime fatigue, cognitive fog, and digestive issues, because their core body temperature and hormone secretions are still operating on home time. A jet lag calculator, by contrast, addresses the root cause of the desynchronosis by actively shifting the biological clock, ensuring that natural sleep and wakefulness are permanently restored.

Another frequent alternative is the use of stimulant countermeasures, primarily heavy caffeine consumption or prescription stimulants like Modafinil. While stimulants are highly effective at suppressing homeostatic sleep pressure (the buildup of adenosine in the brain), they are a masking agent. They allow a traveler to power through a daytime meeting in a new time zone, but they frequently exacerbate the underlying jet lag by remaining in the bloodstream and destroying the traveler's ability to sleep during the new biological night. By comparing these approaches, the superiority of the jet lag calculator becomes evident: while drugs and stimulants only mask the symptoms of circadian misalignment, the mathematical scheduling of light and dark provided by a calculator actually cures the misalignment, offering a biologically sustainable solution without chemical hangovers or dependency.

Frequently Asked Questions

How accurate are the schedules generated by a jet lag calculator? The schedules generated by a high-quality jet lag calculator are mathematically precise, based on decades of peer-reviewed chronobiological research and the universally accepted Phase Response Curve to light. However, the practical accuracy depends entirely on the accuracy of the data inputted by the user, specifically their habitual sleep and wake times. If a user inputs the time they wish they slept, rather than their actual biological sleep schedule, the calculator will miscalculate their Core Body Temperature Minimum, rendering the resulting light exposure schedule ineffective or even counterproductive. When accurate inputs are provided and the user strictly adheres to the light/dark instructions, the calculators are highly accurate, capable of shifting the human clock by 1 to 1.5 hours per day.

Do I absolutely have to use melatonin for the calculator to work? No, the use of synthetic melatonin is not strictly mandatory for a jet lag calculator to successfully shift your circadian rhythm. Light is by far the most powerful zeitgeber (time-giver) for the human biological clock, and manipulating light exposure alone is sufficient to achieve full circadian adaptation. However, incorporating micro-doses of melatonin (0.3mg to 0.5mg) at the exact times specified by the calculator acts as a synergistic signal to the brain, effectively tricking the body into initiating the biological night. Using melatonin in conjunction with light therapy can accelerate the total adaptation rate by approximately 20% to 30%, which is highly beneficial for trips crossing more than six time zones.

Why does the calculator tell me to wear sunglasses indoors during the day? The calculator instructs you to wear sunglasses or avoid light during specific daytime hours because your internal biological clock is still operating on your home time zone. If you have traveled East, the local morning sun may be rising before your internal Core Body Temperature Minimum has occurred. Exposing your eyes to bright light before this critical biological inflection point will cause a Phase Delay, pushing your internal clock backward and actively worsening your jet lag. Wearing dark sunglasses prevents this harmful light from reaching the intrinsically photosensitive retinal ganglion cells in your eyes, protecting your circadian rhythm until the mathematically safe time to seek light arrives.

What happens if I cross more than 12 time zones? When you cross exactly 12 time zones, your internal biological clock is 180 degrees out of phase with the local environment, meaning your biological midnight aligns with local noon. In these extreme scenarios, jet lag calculators face a mathematical fork in the road, as shifting 12 hours forward (East) or 12 hours backward (West) takes the exact same amount of time. Because the intrinsic human circadian rhythm is slightly longer than 24 hours (averaging 24.2 hours), it is biologically easier for the body to delay rather than advance. Therefore, calculators will almost universally default to a massive Westward phase delay protocol, instructing you to stay awake later and sleep later incrementally until you align with the new time zone.

Can I use a jet lag calculator for a weekend trip? Using a full adaptation schedule for a trip lasting less than 48 to 72 hours is generally highly discouraged by chronobiologists. Because the human body can only shift its clock by 1 to 1.5 hours per day, you will not have enough time to fully adapt to the destination before you have to turn around and fly back. Attempting to shift will leave you in a state of perpetual biological transit, resulting in severe "double jet lag" upon your return home. For short trips, advanced calculators will recommend a "compromise phase" strategy, advising you on how to strategically anchor your circadian rhythm to your home time zone while using caffeine and strategic naps to survive the social demands of the destination.

Why does the calculator recommend such a tiny dose of melatonin? Most over-the-counter melatonin supplements are sold in massive pharmacological doses ranging from 5mg to 10mg, which is 10 to 30 times higher than the amount naturally produced by the human pineal gland. Jet lag calculators recommend a physiological dose of 0.3mg to 0.5mg because this specific amount is required to trigger a phase shift in the Suprachiasmatic Nucleus without causing receptor spillover. Massive doses flood the brain, causing the melatonin to remain in the bloodstream well into the next day, which blurs the circadian signal, causes severe grogginess, and actually hinders the phase-shifting process. The goal is to send a sharp, precise chemical signal, not to medically sedate the brain.

Does sleeping on the airplane help or hurt my jet lag? Whether sleeping on the airplane helps or hurts depends entirely on the timing of the sleep relative to the schedule generated by the jet lag calculator. If the calculator indicates that the duration of the flight coincides with what should be your new biological night, then sleeping on the plane is critical and highly beneficial. However, if the flight occurs during what the calculator has designated as your active, light-exposure phase, sleeping on the plane will deprive your eyes of the light necessary to shift your clock, severely delaying your adaptation. You should sleep on the plane only when the mathematical schedule dictates it is time to do so.

How do I handle the light exposure rules if it is raining or overcast at my destination? Natural outdoor light, even on a heavily overcast or rainy day, is significantly more powerful than standard indoor lighting. A heavily overcast sky still delivers between 1,000 and 2,000 lux of light intensity to the eye, whereas a brightly lit hotel room or office rarely exceeds 300 to 500 lux. Therefore, when the calculator instructs you to seek light, spending time outdoors on a cloudy day is still vastly superior to staying inside. If outdoor access is impossible or if the sun has already set locally when you need light, experts rely on portable, medical-grade circadian light boxes that emit 10,000 lux of blue-enriched light, positioning them 16 inches from the face to simulate the biological effects of the sun.