Time zones and jet lag

Feeling sleepy during the day and wide awake at night after a flight across several time zones are classic signs of jet lag. This guide explains how time zones work, why daylight saving time creates shifting differences, why travelling east is harder, and what you can do to help your body adjust to the new time faster. At the bottom of the page you'll find the UTC offset of every country's capital.

Last updated: November 2026

In brief

• Times around the world are expressed relative to UTC (Coordinated Universal Time); for example, UTC+3 means three hours ahead of UTC.
• The body clock can usually shift by about an hour a day, or a little more; with large differences, full adjustment can take several days.
• For most people, travelling east is harder than travelling west.
• Light is the strongest signal for resetting the body clock.

How do UTC and time zones work?

In theory, the Earth is divided into 24 time zones, each 15 degrees wide. In practice, the boundaries follow political decisions: China uses a single time despite its vast territory, while the US and Russia span several zones.

Some countries use half-hour or quarter-hour offsets instead of whole hours. India (UTC+5:30), Nepal (UTC+5:45) and Iran (UTC+3:30) are examples. Watch out for these fractions when reading the offsets in the table.

Why is daylight saving time so confusing?

Most of Europe, North America and some other countries move their clocks forward an hour in summer. But the changeover dates differ: the European Union switches on the last Sunday of March and October, while the US and Canada switch in March and November. As a result, the difference between two cities can shift by an hour for a few weeks of the year.

In the Southern Hemisphere, daylight saving time falls during the northern winter, as in some Australian states, New Zealand and Chile. Many countries don't observe daylight saving time at all. When planning flights and meetings, check the local time at each end separately. The values in the table show the capital's standard time.

What causes jet lag?

Our internal body clock (circadian rhythm) regulates sleep, appetite, body temperature and hormones on a roughly 24-hour cycle. When a fast flight takes you across several time zones, the daylight outside falls out of sync with your internal clock.

Symptoms include insomnia or daytime drowsiness, poor concentration, digestive problems and fatigue. They usually become noticeable after crossing three or more time zones.

Why is travelling east harder?

Most people's internal clock runs slightly longer than 24 hours. That's why lengthening the day, that is, flying west and going to bed later, is easier for the body. When you fly east, you have to go to bed before you feel sleepy and wake up while your body still thinks it's night, which slows adjustment.

With very large differences (eight hours or more), the body sometimes adjusts by shifting backward rather than forward, which can mean a day or two of extra fatigue.

Practical tips for adjusting faster

• Get light at the right time — If you've flown east, get out into the morning light and avoid bright light in the evening. If you've flown west, spend time outdoors in the afternoon and early evening.
• Shift your sleep schedule in advance — A few days before the trip, move your bedtime and wake-up time by half an hour to an hour each day in the direction you're travelling.
• Switch to local time on the plane — At take-off, set your watch to your destination's time and plan your sleep and meals accordingly.
• Drink plenty of water — Cabin air is dry. Drink water, and limit alcohol and evening caffeine.
• Take a short nap on arrival — If you're very tired during the day, a short 20–30 minute nap is enough; a long daytime sleep disrupts night-time sleep.
• Eat meals on local time — Mealtimes also give cues to your body clock.

What to do on short trips

On a two- or three-day business trip, your body may not be able to adjust fully. In that case, it may make more sense to schedule important meetings for when you're most alert according to your home time.

If sleep problems persist, or you need to adjust the timing of medication you take regularly, consult a doctor before you travel. For sleep aids and products such as melatonin, also discuss dosage and suitability with a healthcare professional.

UTC offsets of capital cities by country