Why jet lag is harder when flying east

In adults, the internal clock runs on average on a cycle slightly longer than 24 hours, about 24 h 10 min in a landmark study under controlled conditions. Every day, light resets it to the 24-hour day.

This slight slowness has a direct consequence for travel: it is more natural for the clock to shift a little later than to shift earlier.

When flying west, the travel day gets longer. The clock has to be delayed, that is, move in the direction it already drifts naturally. Adaptation is generally faster.

Flying east, the clock must instead be advanced: falling asleep and waking up earlier than the body expects. This is harder, and falling asleep in the evening at the destination is often the first problem. For very large eastward shifts, the clock sometimes even resets the other way, by delaying many hours.

• the number of time zones crossed and the direction of travel;
• the light you get at your destination, and above all when;
• sleep schedules before and during the trip;
• individual differences, including age and chronotype.