Why is jet lag worse when you fly east?

Jet lag is worse after flying east because your body clock has to shift earlier, and it is built to run slightly late. The master clock in your brain's hypothalamus ticks a little longer than 24 hours, about 24 hours and 11 minutes, and gets nudged back into line every day by outside cues like light. Fly west and you need a phase delay: going to bed and waking up later, which suits a clock that already leans that way. Fly east and you need a phase advance: sleeping and waking earlier than your body wants. Most travellers find the eastward trip harder.

Jet lag is not just tiredness from the trip. It is a mismatch between your internal rhythm and the time zone you are in. Land in London from New York and your body feels as if it were five hours earlier than the clocks around you, so you lie awake at bedtime. After flying west the typical problem flips: you wake far too early. On average, it takes about one day per hour of time difference for the clock to fully catch up.

The feeling is older than its name. The aviator Wiley Post wrote about the effects of flying across time zones in a 1931 book, Around the World in Eight Days, but the phrase 'jet lag' first showed up in the Los Angeles Times in 1966, once jets made such fast trips possible. And it is direction, not distance, that counts: a ten-hour flight from Europe to southern Africa causes no jet lag at all, because it runs north to south.