Jet lag is a sleep disorder caused by a mismatch between your body’s internal clock and the time zone you’ve traveled to. Formally called circadian desynchrony, it kicks in after flying across at least two time zones and typically lasts 1 to 1.5 days for every time zone you cross. So a flight from New York to London (five time zones) can leave you feeling off for nearly a week.
How Your Internal Clock Works
Your body runs on a roughly 24-hour cycle controlled by a tiny cluster of brain cells called the suprachiasmatic nucleus, located deep in the hypothalamus. This cluster acts as a master clock, coordinating when you feel sleepy, when your body temperature drops, when hormones release, and when your digestive system expects food. One of its most important jobs is regulating melatonin, the hormone that signals darkness and sleep.
The relationship is a two-way street: the master clock tells the pineal gland when to produce melatonin, and melatonin feeds back on the clock to fine-tune its timing. This feedback loop is most sensitive during the biological dusk period, which is why evening light and evening melatonin have such powerful effects on your sleep cycle. When you fly across several time zones in a matter of hours, the external cues (sunlight, meal times, social schedules) suddenly don’t match the rhythm your clock has been running on for weeks or months. Your clock can only shift by about an hour or so per day, which is why recovery takes multiple days.
Symptoms Beyond Tiredness
Jet lag is more than just feeling groggy. The CDC classifies it as a disorder with a specific set of symptoms that appear within one to two days of travel. These include:
- Sleep disturbances: difficulty falling asleep, waking up too early, or both
- Excessive daytime sleepiness: feeling heavy and drowsy during hours you need to be alert
- Cognitive impairment: trouble concentrating, slower reaction times, foggy thinking
- Gastrointestinal problems: nausea, constipation, or general stomach discomfort
- General malaise: a vague sense of feeling unwell that’s hard to pin down
These symptoms don’t all hit at once or with equal intensity. You might sleep fine but have stomach issues, or feel mentally sharp but unable to stay awake past 4 p.m. The combination depends on how many time zones you crossed, the direction you traveled, and your individual biology.
Why Eastward Travel Feels Worse
Flying east is harder on the body than flying west. When you travel west, your day gets longer, and your clock needs to delay, which aligns with your body’s natural tendency to drift slightly past 24 hours. Traveling east forces your clock to advance, essentially shortening your day, and that’s a harder adjustment. A six-hour eastward shift (like flying from the U.S. East Coast to Europe) demands your body fall asleep and wake up hours earlier than it’s used to, which is the more difficult direction for the circadian system to shift.
Age and Individual Susceptibility
Not everyone experiences jet lag the same way. In a controlled study simulating a six-hour time shift, early middle-aged men had significantly worse outcomes than younger men. They woke up more during the night, ended sleep earlier than intended, and reported larger drops in alertness and well-being. They also described needing more effort to get through daily tasks. Interestingly, the underlying body temperature rhythm shifted at the same rate in both groups, suggesting the problem wasn’t that older adults’ clocks adjusted more slowly, but that they had more difficulty maintaining sleep when their clock was out of sync.
Your natural chronotype matters too. If you’re already a night owl, eastward travel will feel especially punishing because you’re being asked to sleep even earlier than your body prefers. Early risers tend to handle eastward flights a bit more gracefully but may struggle more heading west.
Using Light to Reset Your Clock
Light is the single most powerful tool for shifting your circadian rhythm. Bright light in the morning pushes your clock earlier (helpful after eastward travel), while evening light pushes it later (helpful after westward travel). Research shows that even 30 minutes of bright light right after waking can meaningfully shift your rhythm, and longer exposure of one to two hours produces even larger shifts. The key finding is that light applied earlier in the circadian morning produces bigger advances than light applied later.
Equally important is avoiding light at the wrong time. If you’re trying to shift your clock earlier, exposure to bright light in the evening hours can undo your progress by pushing the rhythm in the opposite direction. Practical strategies include dimming indoor lights in the hours before your target bedtime, wearing sunglasses if you’re outside during that window, and using blackout shades to keep your sleeping environment dark. This combination of seeking light at the right time and blocking it at the wrong time is what makes the difference between a quick adjustment and a sluggish one.
How Melatonin Supplements Help
Melatonin is the most studied supplement for jet lag, and the evidence is strong. A Cochrane review found that eight out of ten trials showed melatonin reduced jet lag symptoms for flights crossing five or more time zones. Doses between 0.5 and 5 milligrams were similarly effective at shifting the clock, though 5 milligrams helped people fall asleep faster. Doses above 5 milligrams didn’t add any benefit. Starting with 2 to 3 milligrams is a reasonable approach for most people.
Timing matters more than dose. Melatonin should be taken at bedtime after darkness falls on the first day of travel, then again at the same time for the next few nights at your destination. Taking it too early in the day can backfire, causing unwanted sleepiness and actually delaying your adjustment. Taking it before the day of travel doesn’t help either and isn’t recommended.
Meal Timing, Hydration, and Pre-Trip Prep
Your digestive system has its own circadian rhythms, which is part of why jet lag causes stomach problems. Shifting your meal times toward your destination’s schedule, even partially, can help nudge your body clock in the right direction. For short trips across one or two time zones, simply eating, sleeping, and waking a little earlier (for eastward travel) or later (for westward travel) in the days before departure may be enough to prevent noticeable symptoms.
For longer trips, a more deliberate approach helps. In the days before departure, gradually shift your bedtime and meal times by 30 to 60 minutes per day toward your destination’s schedule. During the flight, drink plenty of water and avoid caffeine and alcohol, both of which promote dehydration and worsen jet lag symptoms. There’s also some evidence that a brief period of fasting may help trigger a faster circadian reset, though this mechanism is better established in animal research than in human trials.
How Long Recovery Actually Takes
The general rule is 1 to 1.5 days of recovery for each time zone crossed. A three-zone trip might resolve in three to five days, while a transatlantic flight spanning seven or eight zones could leave lingering effects for over a week. Individual variation is significant: some people bounce back in half that time, while others, particularly older adults, may take longer. The direction of travel, your use of light and melatonin, and whether you pre-adjusted before the trip all influence how quickly you feel normal again.
Sleep disturbances usually resolve first, while cognitive sharpness and digestive comfort can lag behind by a day or two. If you have important meetings or events at your destination, building in at least one buffer day before you need to perform at your best makes a real difference.