Bright Light

How Light Resets Jet Lag: The Science of Getting Your Body Clock Back in Sync

August 3, 2026 15 min read

Anya Ratnavel

Contents

How Light Resets Jet Lag

Most jet lag advice comes down to a few familiar instructions: change your watch to destination time, sleep on the plane, get outside when you land. Most of the advice tends to overlook the most important signal for resetting jet lag: timed light exposure before and after your flight.

Jet lag is essentially a temporary mismatch between your internal body clock and the time where you've landed. A bad night's sleep and an uncomfortable journey can make it worse, but jet lag itself is a problem of timing.[1]

Your circadian system runs the daily rhythms of sleep and wakefulness, alertness, body temperature, digestion, metabolism and hormones. Cross several time zones faster than it can keep up, and the clocks around you change at once while the clock inside you does not.[1] This is why you can be exhausted at midday, wide awake at midnight, and foggy exactly when you need to be sharp. Your body is still living in the original time zone.

The most powerful signal for resetting that clock is light. Using light well doesn't just mean you get as much light as you can as soon as you land. You need the right light, at the right biological time. And after some trips, particularly big journeys east, the usual advice to "get morning light" can push your clock the wrong way.

Three things worth knowing

Jet lag is a timing problem. Your schedule and local time have moved faster than your body clock can follow.
Bright light can move your body clock in either direction. Depending on when it reaches your eyes, it can start your biological day earlier or later.
Local time is not the same as biological time. Eight in the morning where you land can still be the middle of your biological night. Recovering from jet lag means gradually bringing the two back into alignment.

Your body does not run on exactly 24 hours

Your master clock sits in the suprachiasmatic nucleus, or SCN, a small region of the hypothalamus. It keeps its own rhythm even with no clocks, alarms or daylight to go on. Research has shown that the average human circadian period is about 24.18 hours, though it varies from person to person.[2]

Because that internal day is not exactly 24 hours, it has to be nudged back into line with the outside world continually, which is known as entrainment. Food, activity and social routines all feed in, but light is the dominant signal, and light around the edges of your biological night and day matters most of all, because it tells the brain whether to run earlier or later.[3]

In ordinary life these are tiny daily corrections. Fly across time zones and the correction needed jumps to five, eight, even twelve hours at once, and the clock simply cannot move that far that fast. It also moves unevenly. Clinical guidance puts adaptation at roughly 1.5 hours a day after westward travel and closer to one hour a day after eastward travel, though the real rate depends on you, the trip, your light exposure and how many zones you cross.[1]

How your eye tells your brain the time

Your eye does more than just let you see. Alongside the rods and cones you see with, the retina holds a small group of cells called intrinsically photosensitive retinal ganglion cells, or ipRGCs. They contain a light-sensitive pigment, melanopsin, and they send timing information to the parts of the brain that run the body clock, alertness and sleep.[4]

We owe much of this understanding to work in the 1990s by Professor Russell Foster, advisor to Sunday Light, who showed that circadian responses to light could survive without normal rod and cone function, pointing to another light-sensing system.[3] In 2002, Berson et al. identified these photosensitive cells directly.[4][5]

The impact of light on your body clock depends on a number of factors including its whole spectrum, its brightness, how long it lasts, when it arrives, the light you had earlier and your own sensitivity. Melanopsin responds most strongly to blue-cyan light, near 480 nanometres. Colour temperature alone does not capture it either: two lights that both look like 6,000 K can deliver quite different circadian stimulation. This is why circadian research increasingly measures melanopic equivalent daylight illuminance, or melanopic EDI.[6]

How bright does the light need to be?

Your circadian system responds across a wide range, and even modest indoor light can nudge it, especially at night. The response simply grows stronger with more biologically effective light reaching your eyes, and with longer exposure.

In 2022, an international group of circadian and lighting scientists recommended that healthy adults on a normal daytime schedule get at least 250 lux melanopic EDI at the eye during the day, and far less in the three hours before sleep.[6] That daytime figure describes a healthy indoor environment.

What it does expose is a problem with most buildings. Lighting standards are built around helping us see, and are usually measured on a desk rather than at the eye. Even an office that meets them can deliver under 200 lux melanopic EDI to the person sitting in it, and homes are often dimmer still. Outdoor daylight, meanwhile, runs into the thousands or tens of thousands of lux. Your eyes adapt so smoothly that a room can feel perfectly bright while giving your clock a far weaker daytime signal than the outdoors we evolved under.

For jet lag that gap matters, because knowing the right light window is of little use if the light you can reach in it is weak.

When you need the light signal: CBTmin

Which way light moves your clock depends mostly on when it arrives relative to your internal timing. A useful landmark is the daily low point in your core body temperature, or CBTmin.

Body temperature follows its own daily rhythm, falling through the biological night, hitting a low towards the end of this, and then rising in the hours before you wake up. For someone on a steady schedule, CBTmin tends to land about two to four hours before their usual wake time, so roughly 3am to 5am if you wake at 7am.

CBTmin is useful because it sits close to the point where the effect of light changes direction on the human phase-response curve.[7] Light in the hours before and after this point can produce substantial shifts, but in opposite directions.

Generally:

Light in the night, before CBTmin, delays the clock and pushes sleep later.
Light in the morning, after CBTmin, advances it and pulls sleep earlier.
Light in the middle of your biological day does less to timing, though it still lifts alertness.

Intensity, duration and spectrum shape how big the effect is, but timing largely sets its direction. Which is why "get morning light" is not a full protocol.

Why east is usually harder than west

Travelling west means your clock needs to move later, a phase delay. Travelling east means it needs to move earlier, a phase advance.

Because the average human day runs a little longer than 24 hours, most of us find delaying easier than advancing: staying up later goes with the natural drift, while sleeping and waking earlier fights against it.[2] That is why the same distance east often takes longer to shake off than west.

This isn't universal. Circadian period, chronotype, age, sleep habits and light sensitivity all vary, and studies have found more than tenfold differences between people in their circadian sensitivity to the same evening-light exposure.[11]

Very large trips add another wrinkle. When the time difference nears half the day, your clock could in theory reach the destination by advancing one way or delaying the other, and simple rules like "east means morning light" stop being reliable. The plan then has to start from your estimated biological time and move with it.

A worked example: London to Los Angeles and back

London and Los Angeles are usually eight hours apart. Take someone who wakes at 7am, with CBTmin near 4am.

Going out west, the clock needs an eight-hour delay. At 8pm in Los Angeles their body still reads about 4am London time, so they fade early in the evening and wake far too early the next day. To encourage a delay, the useful light is in Los Angeles late afternoon and evening, while keeping the very early local morning dim so it doesn't pull the other way. That window then moves later as the clock catches up.

Coming home, east, is the harder leg, and this is where being aware of CBTmin is critical. Say you have been in California long enough to fully adjust: your CBTmin, now near 4am LA time, falls around midday in London.

On the first day back, London's morning is still before their temperature minimum, so bright light then can delay the clock when they need it to advance. The better opening move is to keep the early London morning dim and seek stronger light after CBTmin, around midday or early afternoon, bringing that window earlier each day as the clock advances.

This is why the right plan for jet lag can be complicated and nuanced, as it depends on your biological time, how far you had already adjusted and the exact trip. Someone back after three days will not be where someone back after three weeks is.

A practical jet lag protocol using light

Work out the shift. Note how many zones you are crossing, whether your clock needs to advance (east) or delay (west), your usual sleep and wake times, and how long you are staying. For a quick one or two day trip, staying close to home time is sometimes easier than adjusting at all.
Start before you go. If you can, move your sleep towards destination time over the 2-5 days before you fly, about 30 to 60 minutes a day rather than all at once. For eastward trips, shift earlier and seek strong light on waking; for westward, shift later and seek light in the late afternoon and evening.[9][10]
Travel on destination time. Once you are moving, arrange sleep, waking and light around the time where you are going. Try not to start already sleep deprived, since that adds a layer of tiredness symptoms on top of the timing problem. After arrival, short naps help; long daytime sleep tends to backfire.
Moderate trips: follow the simple rule. Heading west, seek strong light in the local late afternoon and early evening and keep the early morning dim. Heading east across a moderate shift, seek light after your biological night, which usually falls in the local morning, and keep the evening dim. Move the windows with you as you adapt.
Large eastward trips: plan around your CBTmin. After a big jump east, don't assume local morning is your biological morning. Estimate where CBTmin lands in destination time, keep light dim before it, seek it after. Because a mistimed dose can shift you the wrong way, trips of around seven or more zones are worth an individual schedule rather than a fixed east-or-west rule.[8]

Advice is one thing. Having the right light is another.

A good jet lag plan can tell you exactly when to seek light and when to avoid it. What it can't do is make sure the light is actually there. Your most important window might fall before a winter sunrise, or while you are in a dim office, a hotel room or a meeting. You can know you need a strong daytime signal and have nothing brighter than a normally lit room to hand.

Outdoor daylight at the right time is usually your strongest and simplest option. Indoors, what reaches your eyes is what counts, and the light on a desk is not the light arriving at eye level. You need genuinely bright light around you while you sit, work or eat, and a meaningfully dim one in your avoidance window. As a floor for daytime, 250 lux melanopic EDI at the eye is a sound evidence-based target, and a deliberate phase shift may need considerably more, especially if your window is short.[6]

Sunday Light for jet lag management

That is the gap we built Sunday to close. Sunday has been built to recreate daylight indoors, delivering high-intensity, high-quality light into the spaces where you already live and work.

Its 34,500-lumen output can reach up to 10,000 lux at 60 centimetres, and its colour temperature tunes from a bright 6,000 K by day to a warm 2,650 K in the evening, with a colour-rendering specification of 97+ CRI.

This is key to jet lag management and shifting your body clock, so that you can truly control the light you're getting at various stages of the day, without having to rely on outside weather or a bright light box shining directly on your face.

The Sunday app's Jet Lag Assistant turns the principles set out above into an automated day-by-day plan built around your trip and your usual sleep times.

Most jet lag tools can tell you when you need the right light. We built Sunday to give you the light itself.

Frequently asked questions

What is jet lag? Jet lag is a temporary mismatch in what your biological body clock is running on compared to the new time zone you have arrived in. Your internal time stays closer to where you left while your schedule has jumped to where you landed, which brings broken sleep, which leads to a number of symptoms including tiredness.

How long does jet lag last? Average circadian adaptation is approximately one hour per day after eastward travel and around 1.5 hours per day after westward travel. These are rough population averages: the number of time zones crossed, sleep loss, light exposure and individual biology all affect how quickly you adjust.[1]

Is jet lag different from travel fatigue? Yes. Travel fatigue comes from sleep loss, long hours sitting, stress and disrupted routines, and can follow even a long north to south flight that crosses no time zones. Jet lag is specifically the mismatch between internal and local time. They often arrive together, but they are not the same thing.

Is jet lag worse going east? Usually, but not for everyone. East generally means advancing your clock, west means delaying it, and since the average human day runs slightly over 24 hours, delaying is easier.

Will getting morning light always help? No. Morning light usually advances the clock, but the local morning right after a big eastward trip can still fall before your core body temperature minimum (CBTmin), where light delays you instead. What matters is where the light lands relative to your biological time, rather than the time in your local time zone.

How bright should the light be? For a healthy daytime indoor environment, expert consensus suggests at least 250 lux melanopic EDI measured at the eye. Deliberate phase-shifting usually calls for brighter light, with the result depending on timing, spectrum, duration and the individual.

Can ordinary indoor light affect my body clock? Yes, its effect depends on factors including the spectrum, timing, duration and the amount of light reaching your eyes. . Many rooms provide a much weaker daytime signal than outdoor daylight, so ordinary indoor light will generally produce a smaller circadian effect than a brighter, well-timed exposure

Can a SAD lamp help with jet lag? Yes, a properly specified bright-light box, used at the correct times for your travel, can shift circadian timing. The difference between Sunday Light and a bright light box, is that with a bright light box or SAD lamp, you need to sit very close to the lamp with light shining directly in your face to get the right amount of light. As an alternative, Sunday Light offers a more passive way of receiving this light across your space, allowing bright-light exposure to become part of your normal environment rather than a separate ritual.

Can I start adjusting before I travel? Yes. Moving your sleep and light gradually towards destination time for 2-5 days before you leave reduces how much your clock still has to shift on arrival. It helps most before eastward trips, where advancing is the slower direction.