What happens to the body when changing time zones?
When you fly across several time zones, your body's internal clock — the circadian rhythm that controls sleep, hunger, body temperature and hormones — keeps running on home time. The mismatch is called jet lag. It typically causes trouble sleeping at night, sleepiness during the day, difficulty concentrating, irritability and digestive problems, and usually fades within a few days as your clock catches up.
Why it happens
Your internal clock resets mainly through light, and secondarily through meals, activity and social cues. It can shift only about an hour or so per day, so after a large jump it lags behind local time. A common rule of thumb is that full adjustment takes about one day per time zone crossed, though many people feel fine sooner.
Flying east is usually harder than flying west, because it means going to bed earlier than your body wants to; it's easier to stay up late than to fall asleep early.
Common symptoms
- Insomnia or waking very early
- Daytime drowsiness and fatigue
- Headache and trouble focusing
- Indigestion, constipation or loss of appetite
- General malaise and mood changes
Travel fatigue — from a long flight, dry cabin air and little sleep — adds to these symptoms, even on north–south flights without time change.
How to adjust faster
- Shift before you go. For a few days before an eastward trip, go to bed and wake up a bit earlier; for a westward trip, a bit later.
- Use daylight. After flying east, get morning light; after flying west, get afternoon and evening light.
- Switch to local time immediately. Eat meals and go to bed on the new schedule.
- Nap carefully. A short nap of 20–30 minutes is fine; long naps delay adaptation.
- Drink water, limit alcohol, use caffeine only in the morning.
- Ask a doctor about melatonin or other aids if you travel often across many time zones.
FAQ
What is the medical name for jet lag?
Jet lag disorder, also called desynchronosis or a circadian rhythm sleep disorder of the jet lag type.
How long does it take to adjust to a new time zone?
Roughly a day per time zone crossed, but often less with good light exposure and sleep habits.
Can jet lag make you feel sick?
Yes. Nausea, stomach upset and headache are common, especially in the first days.
Why do I feel sleepy in a different time zone?
Your body clock still follows home time, so it releases sleep signals at what is now the wrong hour. Daylight and local meal times help it reset.
Is it harder to go ahead a time zone?
Usually yes. Moving clocks forward, as when flying east, means sleeping earlier than your body wants, which most people find harder than staying up later.
How long does it take to adjust after a long trip, such as India to Toronto?
That is about a 10-hour difference, so full adjustment can take a week or more, though most people feel mostly normal within a few days.