You land in London after a seven-hour flight from New York. Your phone says it is 10 PM, but your body insists it is 5 PM and dinner was supposed to be an hour ago. Over the next few days, you will fall asleep at odd hours, wake up at 3 AM feeling wired, and struggle to concentrate during afternoon meetings. This is jet lag, and it is not just about being tired. It is a genuine disruption of your body's internal clock.
Your circadian rhythm, explained briefly
Every cell in your body runs on a roughly 24-hour cycle called the circadian rhythm. This internal clock is managed by a tiny cluster of neurons in your brain called the suprachiasmatic nucleus (SCN), located just above where the optic nerves cross. The SCN takes its primary cue from light entering your eyes. When light hits your retina in the morning, it signals the SCN to suppress melatonin production and ramp up cortisol, waking you up. As light fades in the evening, melatonin rises and you feel sleepy. This cycle regulates not just sleep but also body temperature, hunger, hormone release, and cognitive performance. Your digestive system, liver, and even your skin have their own peripheral clocks that take cues from the SCN. When you cross time zones, these peripheral clocks can take different amounts of time to adjust, which is why jet lag affects more than just sleep.
What happens when you cross time zones
When you fly across multiple time zones, you arrive in a place where the local light-dark cycle is out of sync with your internal clock. Your SCN is still running on departure-city time. The result is a mismatch: your brain wants to sleep when the local sun is up, and wants to be alert when everyone around you is heading to bed. Your body can adjust, but it takes time. The general rule of thumb is about one day of adjustment per time zone crossed, though this varies by individual and direction of travel. North-south flights (say, New York to Buenos Aires) generally do not cause jet lag even if they are long, because you stay within the same timezone band. It is specifically the east-west shift, the mismatch between your internal clock and local solar time, that triggers the symptoms. Even a three-hour shift (like New York to Los Angeles) is enough to disrupt sleep for a night or two, though most people barely notice differences of one or two zones.
Why eastward travel is worse
Most people find it harder to fly east than west. The reason is biological. The human circadian rhythm is actually slightly longer than 24 hours (closer to 24.2 hours on average). This means your body naturally tends to drift later, not earlier. When you fly west, you are extending your day, which aligns with this natural tendency. When you fly east, you are shortening your day, forcing your clock to advance, which works against the grain. A study published in the journal Chaos in 2016 mathematically modeled this asymmetry and found that recovery from eastward travel takes about 50% longer than from an equivalent westward trip. This is why a London-to-New York flight (westward, gaining 5 hours) typically feels more manageable than the return trip, even though they cover the same distance. Athletes competing internationally often arrive several days early for eastward events to give their bodies time to adjust, a practice backed by sports science research showing measurable performance impacts from circadian misalignment.
Evidence-based recovery strategies
Light exposure is the most powerful tool for resetting your circadian clock. After flying east, get bright light in the morning to advance your clock. After flying west, seek light in the late afternoon and evening. Avoid bright light (especially blue light from screens) during the hours when your body thinks it should be dark. Melatonin supplements can help if timed correctly: take 0.5 to 3 mg about 30 minutes before your desired bedtime in the new timezone. Some travelers begin shifting their sleep schedule by 30 to 60 minutes per day for several days before the trip, which can reduce the shock on arrival. Staying hydrated, avoiding heavy meals during what your body considers nighttime, and getting moderate exercise during daylight hours at your destination also help.
When jet lag gets serious
For occasional travelers, jet lag is an inconvenience. For frequent flyers, flight crews, and business travelers who cross time zones every week, the effects can compound. Chronic circadian disruption has been linked to increased risk of cardiovascular disease, metabolic disorders, weakened immune function, and cognitive decline. Airline pilots and cabin crew are subject to strict rest regulations partly because of these health risks. If your work requires frequent long-haul travel, strategies like maintaining a home-timezone schedule during short trips (when practical) and prioritizing recovery time after timezone shifts become especially important. Some seasoned travelers swear by the "no nap" rule: stay awake until a reasonable local bedtime on your first day, no matter how exhausted you feel. It is brutal in the moment, but it forces your clock to align faster. Ultimately, jet lag is the price we pay for moving faster than our biology was designed to handle.