Jet Lag Recovery Calculator

Jet lag happens when you cross time zones faster than your body clock can follow. Your internal sleep and wake rhythm stays on home time while the sun and the clock say something else³. This calculator estimates how long that mismatch is likely to last, based on how many time zones you cross and which way you fly.

What Jet Lag Recovery Time Measures

It estimates the number of days until your body clock matches local time. It does not measure how tired you feel on a given day. It also cannot predict your personal result, because people differ in how fast they adjust³.

The Formula

The calculator turns your time difference into hours of clock shift, then divides by how many hours the body clock can shift per day. Without a light plan, the clock can delay by up to 1.5 hours a day and advance by up to 1 hour a day. With well timed light, it can delay about 2 hours a day and advance about 1.5 hours a day².

Days (fast end) = clock shift in hours / 2 (west) or 1.5 (east)

Days (slow end) = clock shift in hours / 1.5 (west) or 1 (east)

The faster rates match a widely cited summary that jet lag lasts roughly half a day per time zone after westward flights and two thirds of a day per zone after eastward flights¹. The slower rates allow for no light plan².

How This Calculator Computes Your Result

Enter your home and destination UTC offsets, or type the number of time zones and the direction. If you use offsets, the tool takes the shorter way round the clock. For example, Los Angeles to Tokyo counts as 7 zones west, not 17 zones east. Each day count is rounded up to a whole day, and you see a range such as "about 4 to 5 days". The optional wake time sets your light timing table.

What Your Result Means

Your result is a range. The faster end assumes you follow the light timing guide. The slower end assumes you do not. The size of the time difference also matters. Jet lag is usually noticed once you cross about three or more zones¹.

Very large eastward trips are different. A forward shift of more than 8 hours is hard for the body clock, so the clock often shifts the other way round instead¹ ². This calculator treats eastward trips of 9 or more zones as westward trips, and the estimate may run longer.

Why East Trips Take Longer

Your body clock runs a little longer than 24 hours when left alone, so it finds it easier to run later than earlier¹. Flying west asks it to run later. Flying east asks it to run earlier. In one model of 55,296 simulated flights, jet lag lasted about 1.5 times as long on average when the body clock had to move earlier, though staying awake in daylight at the destination closed most of that gap⁴. Real trips vary a lot, so use the range as a guide and not a promise.

How the Light Timing Guide Works

Light is the strongest signal that moves the body clock. The guide uses your body clock minimum, the low point of your body temperature, which is about 3 hours before your usual wake time². After flying west, seek bright light in the 4 hours before that point and avoid it in the 4 hours after. After flying east, avoid light in the 4 hours before it and seek it in the 4 hours after². The table moves these windows each day as your clock moves. Times are shown in destination local time.

Using local light and dark usually speeds up adjustment for trips of 2 to 10 time zones³.

What Else Changes How Long It Takes

Sleep during the trip, how much local light you get, and personal tolerance all affect the length of jet lag³. Older travellers tend to have worse jet lag, although the reason is unclear¹. This calculator does not include any of these.

Melatonin has been studied for jet lag, and sleep medicine guidelines discuss it⁵. This calculator does not give melatonin advice or doses. If you are thinking about it, a doctor or pharmacist can check it against your own health and medicines.