Jet lag is a temporary sleep disorder that happens when you fly across two or more time zones faster than your internal body clock can adjust. Your brain’s clock shifts by roughly 60 to 90 minutes per day on its own, so crossing six time zones can leave you out of sync for the better part of a week. The result is a predictable mix of poor sleep, daytime fog, and physical discomfort that resolves once your body catches up to local time.
How Your Internal Clock Works
Deep in the brain, a tiny cluster of neurons acts as a master clock, coordinating nearly every biological rhythm in your body: when you feel sleepy, when hormones rise and fall, when your gut expects food, and when your body temperature dips overnight. This clock runs on a cycle slightly longer than 24 hours, so it relies on outside cues to stay aligned with the actual day. The most powerful cue is light.
Specialized cells in your retinas detect light and send signals directly to this master clock. Light at different times of day has opposite effects. Light in the early evening delays your clock, pushing your sleep schedule later. Light in the early morning advances it, pulling your schedule earlier. The switchover point between these two effects happens in the middle of the night, around the time your core body temperature reaches its lowest point and you feel most deeply sleepy.
When you land in a new time zone, the light signals from the local sunrise and sunset no longer match what your clock expects. Different systems in your body also reset at different speeds. Your brain’s master clock adjusts relatively quickly, but the clocks embedded in your digestive system, muscles, and other organs lag behind. This internal mismatch, where parts of your body are on different schedules, is what makes jet lag feel so disorienting.
Common Symptoms
Jet lag typically shows up within a day or two of arrival and affects both your mind and body. The hallmark symptoms are insomnia at night and heavy sleepiness during the day, often with a noticeable drop in total sleep. Beyond sleep problems, most travelers experience some combination of the following:
- Cognitive fog: difficulty concentrating, slower reaction times, and poor short-term memory
- Digestive issues: nausea, constipation, or an upset stomach, because your gut is still running on home-time meal schedules
- General malaise: a vague, run-down feeling similar to the early stages of a mild illness
- Mood changes: irritability, low motivation, or mild anxiety, particularly after long eastward flights
The severity depends on how many time zones you cross. A two or three zone hop might cause mild grogginess for a day. A transatlantic or transpacific flight can leave you struggling for several days.
Why Eastward Travel Is Harder
Most people find flying east significantly worse than flying west, and there’s a clear biological reason. Because your internal clock naturally runs a bit longer than 24 hours, it has a built-in tendency to drift later. Delaying your schedule (which is what westward travel requires) works with that natural drift. Advancing your schedule (what eastward travel demands) works against it.
Studies tracking real travelers found that the body’s clock shifts about 92 minutes per day after westward flights but only about 57 minutes per day after eastward flights. In practical terms, that means a six-hour westward trip might take three to four days to fully adjust to, while the same trip eastward could take closer to six or seven days.
The Airplane Itself Makes Things Worse
Time zone disruption is the core problem, but the flight environment piles on. Airplane cabins are pressurized to the equivalent of roughly 2,400 meters (about 8,000 feet) of altitude, and the relative humidity hovers between 10 and 20 percent. That’s drier than most deserts.
In those conditions, your body loses water much faster than normal. Respiratory water loss alone can jump from about 160 milliliters per hour at normal humidity to 360 milliliters per hour in cabin-dry air. One study simulating a 10-hour flight found that blood plasma volume dropped 6 to 9 percent, even without exercise. Fluid also tends to shift toward the lower legs during long periods of sitting, which compounds the dehydration effect. Arriving dehydrated, stiff, and mildly oxygen-deprived makes the first day or two of jet lag feel worse than the clock mismatch alone would predict.
How Long Recovery Takes
The general rule is that your clock can shift one to two hours per day under good conditions. So crossing five time zones eastward might take three to five days of adjustment, while the same trip westward might take two to three. These estimates assume you’re getting appropriate light exposure and sleeping on a reasonable local schedule. If you stay indoors or keep erratic hours, recovery can stretch out longer.
For short trips of two or three days, some travelers find it easier to simply stay on home time rather than try to adjust. This works best when you can control your schedule and the time zone difference is modest (three zones or fewer).
Using Light to Reset Your Clock
Strategic light exposure is the most effective tool for speeding up adjustment, because light is the signal your brain’s clock responds to most strongly. The key is getting the timing right.
After flying east (when you need to shift your schedule earlier), seek bright light in the morning and avoid it in the evening. After flying west (when you need to shift later), do the opposite: get light exposure in the evening and avoid early morning light for the first day or two. Outdoor daylight, which typically exceeds 10,000 lux even on a cloudy day, is the simplest source. Bright light therapy lamps rated at 2,500 lux or above are effective for indoor use.
One critical caution: light exposure at the wrong time can push your clock in the wrong direction. After a large eastward shift (eight or more zones), what feels like “morning” at your destination may still be the middle of the biological night for your body. Bright light at that point would actually delay your clock further. If you’ve crossed many zones eastward, it’s worth wearing sunglasses on the first morning and waiting a few hours before exposing yourself to bright light.
Melatonin and When to Take It
Melatonin is the hormone your brain releases in the evening to signal that it’s time to sleep. Taking it as a supplement can help nudge your clock toward the new time zone. A review by the Cochrane Collaboration found that doses between 0.5 and 5 milligrams are effective, with 5 milligrams helping people fall asleep faster. For most travelers, starting with 2 to 3 milligrams is reasonable.
Timing matters more than dose. Take melatonin at bedtime after dark on your first day of travel and continue for a few nights after arrival. Taking it too early in the day can cause drowsiness and actually slow your adjustment. There’s also no benefit to starting melatonin before departure day.
Melatonin is most useful for eastward flights across five or more time zones, where advancing the clock is hardest. For shorter westward trips, many travelers don’t need it at all.
Practical Strategies That Help
Beyond light and melatonin, a few straightforward habits can reduce jet lag severity. Staying well hydrated during the flight counteracts the cabin’s dehydrating effects. Drinking an extra 15 to 20 milliliters of water per hour of flight is sometimes recommended, though research suggests even that may not fully compensate for the moisture you lose. Avoiding alcohol and caffeine in the hours before your target bedtime at the destination helps protect sleep quality on the first night.
Shifting your meal times to match your destination as soon as possible gives your digestive system a time cue to work with. Eating a full meal at local breakfast time, even if your body thinks it’s 3 a.m., helps pull your peripheral clocks into alignment faster. Light exercise on arrival day, such as a walk outside, combines the benefits of light exposure, gentle physical activity, and social engagement, all of which reinforce the new schedule.
For frequent long-haul travelers, gradually shifting sleep and wake times by an hour per day in the days before departure can reduce the gap your body has to close on arrival. This is more practical for eastward trips, where the adjustment is harder, and works best when the trip is planned well in advance.