Jet lag, medically referred to as "desynchronosis," is a physiological condition which is a consequence of alterations to circadian rhythms; it is classified as one of the circadian rhythm sleep disorders.
Jet lag results from rapid long-distance transmeridian (east-west or west-east) travel, as on a jet plane.
The condition of jet lag may last many days, and recovery rates of 1 day per eastward time zone or 1 day per 1.5 westward time zones are mentioned as fair guidelines.
There seems to be some evidence that for most people,
traveling west to east is more disruptive. This may be because most
people have a circadian period which is a bit longer than 24 hours,
making it easier to stay up later than to get up earlier.
It may also be that flights to the east are more likely to
require people to stay awake more than one full night in order to adjust
to the local time zone.
For example, comparing a typical schedule for a
traveller flying to the west vs a traveller flying to the east:
- Westbound from London to Los Angeles, Jan 29,
2008. Time zone difference 8 hours.
Westbound
Biological clock
London local time
Los Angeles local time
Departure
JAN 29 – 10:05
JAN 29 – 02:05
Arrival
JAN 29 – 21:10
JAN 29 – 13:10
Bedtime
JAN 30 – 06:00
JAN 29 – 22:00
- Eastbound from Los Angeles to London, Jan 29,
2008.
Eastbound
Biological clock
Los Angeles local time
London local time
Departure
JAN 29 – 15:50
JAN 29 – 23:50
Arrival
JAN 30 – 02:00
JAN 30 – 10:00
Bedtime
JAN 30 – 14:00
JAN 30 – 22:00
The first scenario is equivalent to staying up all night and
going to bed at 6 a.m. the next day—8 hours later than usual. But the
second scenario (eastward) is equivalent to staying up all night and
going to bed at 2 p.m. the next day—14 hours after the time one would
otherwise have gone to bed. Some sleep onboard may help the situation
somewhat.
The Red-eye flight is another eastward scenario, for example
flights departing the west coast of the US at midnight (PST/PDT) and
arriving on the east coast early in the morning (EST/EDT). Relative to
the shorter flight time and the time zones advanced, the body gets less
than optimal rest to begin a day of activity.
Further Reading
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"Jet lag"
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