Jet lag is a temporary sleep disorder that happens when you fly across two or more time zones faster than your body’s internal clock can adjust. The general rule of thumb is that recovery takes about one day per time zone crossed, though direction of travel, age, and individual biology all influence how long you’ll feel off. A flight from New York to London (five time zones east) might leave you foggy for four to six days, while the return trip west often resolves faster.
What Happens Inside Your Body
Your brain has a master clock, a tiny cluster of cells in the hypothalamus that keeps nearly every function in your body on a roughly 24-hour cycle. This clock synchronizes itself to the local light-dark cycle using specialized cells in your eyes that detect light and send signals directly to the brain. When you land in a new time zone, the light signals suddenly arrive at the wrong times, and your master clock falls out of sync with local day and night.
That desynchronization cascades through your whole body. Your brain’s clock controls when the pineal gland releases melatonin, the hormone that tells your body it’s time to sleep. Melatonin also feeds back to the master clock, reinforcing its rhythm. When the external light cues shift abruptly, melatonin production keeps following the old schedule for days, which is why you feel wide awake at 2 a.m. and crushingly sleepy at lunch. Digestion, body temperature, hormone release, and alertness all run on their own internal timers, and they don’t all shift at the same rate. This internal mismatch is what makes jet lag feel like more than just tiredness.
Common Symptoms
Jet lag goes well beyond feeling sleepy. The CDC classifies it as a disorder with a specific set of symptoms that typically appear within one to two days of arrival:
- Sleep disruption: difficulty falling asleep, waking too early, or both, with a noticeable drop in total sleep time.
- Daytime fatigue and sleepiness: even after sleeping, you may struggle to stay alert during the afternoon or early evening.
- Cognitive impairment: trouble concentrating, slower reaction times, and difficulty functioning at your normal level.
- Digestive problems: constipation, diarrhea, nausea, or reduced appetite, because your gut runs on its own circadian schedule.
- Mood changes: irritability, low motivation, or a general feeling of malaise that’s hard to pin down.
You might experience just one of these or several at once. The severity usually peaks on the first or second day and then gradually improves as your clock catches up to local time.
How Long Recovery Actually Takes
The one-day-per-time-zone estimate is a useful starting point, but it’s rough. A three-zone trip (like Los Angeles to New York) might resolve in two to four days. A larger shift of eight or more zones can take over a week. The complicating factor is that different body systems adjust at different speeds. Your sleep-wake cycle might normalize within a few days while your digestion or alertness patterns lag behind for another day or two.
For most travelers crossing three to six time zones, expect meaningful improvement by day three or four and full adjustment within a week. Crossing eight or more zones can stretch recovery to 8 to 12 days, especially if travel is eastward.
Why Flying East Feels Worse
Direction matters more than most people realize. When you fly west, say from London to Los Angeles (eight time zones), your body clock is running ahead of local time. Your brain thinks it’s 11 p.m. when it’s only 3 p.m. locally. To adjust, your clock needs to delay, essentially staying up later each day. This is relatively natural because the human circadian cycle tends to run slightly longer than 24 hours, so pushing it later is easier than pulling it earlier.
Flying east reverses the problem. Your clock is behind local time, and it needs to advance, meaning you have to fall asleep and wake up earlier than your body wants. This shift against your natural drift is harder, which is why eastbound travelers consistently report worse symptoms and longer recovery. If you’re planning a trip to Europe from the Americas, the outbound leg will almost always hit you harder than the flight home.
Age and Individual Differences
In a controlled study that simulated a six-hour eastward time shift, middle-aged adults had significantly more difficulty than younger adults. They spent more time awake during scheduled sleep periods, woke up earlier than intended, and reported larger drops in alertness and well-being. Interestingly, their core body temperature rhythm shifted at the same rate as the younger group’s, suggesting the problem wasn’t a slower clock but rather a reduced ability to sleep at unfamiliar times in the circadian cycle. The compensatory response to sleep loss also appeared weaker in the middle-aged group.
Beyond age, your natural chronotype plays a role. If you’re naturally a night owl, eastward travel (which requires going to bed earlier) can be especially brutal. Morning types often handle eastward trips somewhat better but may struggle more with westward flights. People who are generally flexible sleepers, those who can nap easily and fall asleep in varied conditions, tend to recover faster regardless of direction.
How Light Exposure Speeds Recovery
Light is the single most powerful tool for resetting your internal clock, because it’s the primary signal your brain uses to calibrate its rhythm. The key is getting light at the right time and avoiding it at the wrong time.
After eastward travel, you want to seek bright light in the morning (local time) and avoid it in the late evening. This pushes your clock earlier, aligning it with the new time zone. After westward travel, the opposite applies: get light in the late afternoon and evening and avoid bright morning light for the first couple of days. Outdoor sunlight is ideal because it’s far more intense than indoor lighting, typically thousands of lux compared to the 60 or fewer you get from a desk lamp.
Getting the timing wrong can backfire. If you expose yourself to bright light at the wrong point in your circadian cycle, you can actually push your clock in the opposite direction, a phenomenon called antidromic re-entrainment. For large time zone shifts (eight or more hours), it’s worth looking up the specific light-seeking and light-avoidance windows for your trip, since the rules become less intuitive.
Pre-Travel Adjustments
Shifting your sleep schedule before you leave can shorten your adjustment period at your destination. For eastward trips, move your bedtime and wake time 30 to 60 minutes earlier each day for three days before departure. For westward trips, shift later by the same amount. Research on preflight adjustment found that combining these schedule shifts with bright light exposure for the first three and a half hours after waking (using either continuous or intermittent bright light above 3,000 lux) produced meaningful phase advances before travelers even boarded the plane.
Even without a light box, you can approximate this by spending your first hours awake near a window or outside. The goal is to get your clock partially shifted so it has less ground to cover once you arrive. Even a two-hour head start can mean one or two fewer days of brain fog at your destination.
Other Strategies That Help
Melatonin supplements can complement light exposure by reinforcing the signal to your clock. Taken in the evening at your destination (for eastward travel), a small dose helps tell your brain that nighttime has arrived. The timing matters more than the dose. Taking melatonin at the wrong point in your cycle can shift your rhythm the wrong way, just like poorly timed light.
A few other practical habits make a difference. Staying hydrated during the flight helps because cabin air is extremely dry and mild dehydration amplifies fatigue. Eating meals on your destination’s schedule as soon as possible gives your digestive clock an additional time cue. Caffeine can bridge alertness gaps during the day but should be avoided within six hours of your target bedtime, or you’ll undermine the very sleep you need to recover. Short naps (under 30 minutes) can take the edge off daytime sleepiness without interfering with nighttime sleep.
Alcohol is worth mentioning because many travelers drink on flights or in the first days at their destination. It might make you drowsy initially, but it fragments sleep and suppresses the deeper stages of sleep your body needs most during recovery. Skipping it for the first two or three days after arrival gives your clock the best chance to reset cleanly.