<P> In general, adjustment to the new time zone is slower for east--west travel than for west--east . A westward adjustment takes, in days, approximately half the number of time zones crossed; for eastward travel, adjusting to the new time zone takes, in days, approximately two - thirds the number of time zones crossed . Studies have shown that performance in both individual and team sports is measurably better in athletes who have flown westward to the venue than in the opposite direction . </P> <P> Travelling east causes more problems than travelling west because the body clock has to be advanced, which is more difficult for the majority of humans than delaying it . Most people have an endogenous circadian rhythm that is longer than 24 hours, so lengthening a day is less troublesome than shortening it . Equally important, the necessary exposure to light to realign the body clock does not tie in with the day / night cycle at the destination . </P> <P> Travelling east by six to nine time zones causes the biggest problems, as it is desirable to avoid light in the mornings . Waterhouse et al. recommend: </P> <Table> <Tr> <Th> Time zones </Th> <Th> Local time to avoid light at destination </Th> <Th> Local time to seek light at destination </Th> </Tr> <Tr> <Td> East 6h </Td> <Td> 0300--0900 </Td> <Td> 1100--1700 </Td> </Tr> <Tr> <Td> East 7h </Td> <Td> 0400--1000 </Td> <Td> 1200--1800 </Td> </Tr> <Tr> <Td> East 8h </Td> <Td> 0500--1100 </Td> <Td> 1300--1900 </Td> </Tr> <Tr> <Td> East 9h </Td> <Td> 0600--1200 </Td> <Td> 1400--2000 </Td> </Tr> </Table>

How long do you have to fly to get jet lag