You have a clock in your head
Deep in the brain, a cluster of about 20,000 neurons keeps a rhythm close to 24 hours long. That's your circadian clock. It decides when you feel sleepy, when your body temperature dips, when hormones like melatonin and cortisol rise and fall, even when your gut expects food. Left alone in a cave it would run a little fast or slow. In normal life it gets reset every day by one signal above all others: light hitting your eyes, especially in the morning.
Jet lag is what happens when you carry that clock across several time zones faster than it can adjust. Your watch says 8 PM in Paris. Your body still says 2 PM in New York. Until the two agree, you get the familiar package: wide awake at 3 AM, wiped out at lunch, hungry at strange hours, foggy and short-tempered.
Why east is harder than west
Most people's internal clock runs a little longer than 24 hours, which means staying up later is easy and falling asleep earlier is hard. Flying west asks you to stretch your day, which is the direction your body already drifts, so it adjusts about a third faster. Flying east asks you to squeeze the day and fall asleep before your body is ready, and it resists. A rough rule: about one day of recovery per time zone going west, and closer to a day and a half per zone going east. New York to Paris is six zones east and can take most of a week to fully absorb.
Light is the lever
Since light sets the clock, the fastest reset comes from getting bright light at the right time and avoiding it at the wrong time. Light in your body's morning shifts the clock earlier. Light in your body's evening shifts it later.
- Flying east, say from the US to Europe: get bright morning light at your destination. If you arrive badly shifted, skip very early light on the first day, because your body may read 7 AM as late night and shift the wrong way. Mid-morning outdoor light is the safe choice, then earlier each day.
- Flying west: stay in bright light as late as local daylight allows to stretch your day. Morning light matters less.
- Screen light counts. Late-night phone use fights your adjustment, so dim the screens in your new local evening.
The rest of the kit
Start before you fly. Moving bedtime and wake time 30 to 60 minutes a day toward the destination for two or three days before departure takes a real bite out of the lag. Use the converter to see what your normal bedtime becomes over there and aim for it. On the plane, set your watch to destination time at takeoff and act like it. Sleep if it's night there (an eye mask is the cheapest jet lag tool that actually works). Stay awake if it's day. Drink water; cabin air is drier than a desert and dehydration makes every symptom worse. Go easy on alcohol, which breaks up the sleep you're trying to protect. When you land, live on local time from the first hour, meals included, because your gut has its own clocks and food helps set them. If you have to nap, keep it under 30 minutes and before mid-afternoon. Melatonin in small doses near the destination bedtime has decent evidence for eastward trips; it works as a darkness signal. Talk to a pharmacist or doctor first, especially if you take other medication.
When not to bother
One or two zones? Your clock absorbs that in a day without help. It's the same shift as daylight saving weekend, and the DST guide explains why even that one hour stings. For a trip of only a couple of days across many zones, experienced travelers often don't adjust at all. They keep home time, schedule around it, and spare themselves two resets in one week. Diplomats and flight crews do this all the time.
Planning the trip: check what time it is there now, when the sun rises and sets there (useful for planning your light), and what weather to pack for.