How your body clock works
Short version: your body keeps a clock that runs a little longer than a day, light is the signal that moves it, and it moves slowly. That is why jet lag takes days, and why you can often handle it yourself.
What jet lag is
Jet lag is a mismatch between your body clock and the clock on the wall. Cross two or more time zones and your sleep, hunger and alertness keep following home time for a few days. It is temporary, and it is not a sign you are bad at travelling.
Why flying east is usually harder
Your body clock runs a little longer than 24 hours, so left alone it drifts later. Flying west asks it to go later, which suits that drift. Flying east asks it to go earlier, which works against it. Going west tends to feel easier than going east.
Light moves the clock
Light is the strongest signal your body clock responds to. Bright light in your morning nudges the clock earlier. Bright light in your evening nudges it later. Light at the wrong time can pull the clock the wrong way, which is why timing matters more than amount.
How fast it moves
The app's Steady pace uses the typical rates: about one hour a day going east and about an hour and a half going west. Fast pace asks for up to half an hour a day more. These are averages. People differ, and the clock does not jump to a new time zone overnight.
What does not move the clock
Caffeine, naps, meals, exercise and water do not do what light does. They change how alert, comfortable and rested you feel, and some are worth using for exactly that. Melatonin is a separate subject: researchers study its timing, and it is not a substitute for light. We show melatonin timing for information only, and you should ask a clinician before taking it.
Do you need a plan?
Often, no.
- A trip of about two days or less: many people do best staying on home time.
- A shift of a few hours: a few days of good habits usually covers it.
- A big shift, a tight schedule, or something that matters at the other end, such as a wedding, a race or a presentation: a plan helps. You can build one yourself from the tips, or have the app build it for you.
Where it gets tricky
After a very large eastward shift, morning light at your destination can arrive before your body clock is ready for it and pull the clock the wrong way for the first days. Working out when to seek light and when to avoid it is the part most people find hard to do by hand. It is the main thing the app does.
Talk to a clinician first if
You have a sleep condition, take medicines that affect sleep or alertness, or have a health condition that makes changes to your sleep risky.