Jet lag, medically referred to as desynchronosis, is a
physiological condition which results from alterations to the body's
circadian rhythms resulting from rapid long-distance transmeridian
(east–west or west–east) travel on a (typically jet) aircraft. It is
classified as one of the
circadian rhythm sleep disorders.
The condition of jet lag may last several days until one is fully
adjusted to the new time zone, and a recovery rate of one day per
time
zone crossed is a suggested
guideline. The issue of jet lag is especially pronounced for airline
pilots, crew, and frequent travelers. Airlines have regulations aimed at
combating pilot fatigue caused by jet lag.
The common term jet lag is used because before the arrival of
the passenger
jet aircraft, it was generally uncommon to travel far and fast
enough to cause jet lag. Propeller flights were slower and of more
limited distance than jet flights, and thus did not contribute as widely
to the problem.
Cause
Jet lag is a
chronobiological-related problem,[1]
similar to issues often induced by shift work. When traveling across a
number of
time
zones, the
body clock will be out of synchronization with the destination time,
as it experiences daylight and darkness contrary to the rhythms to which
it has grown accustomed: the body's natural pattern is upset, as the
rhythms that dictate times for eating, sleeping, hormone regulation and
body temperature variations no longer correspond to the environment nor
to each other in some cases. To the degree that the body cannot
immediately realign these rhythms, it is jet lagged.
The speed at which the body adjusts to the new schedule depends on
the individual; some people may require several days to adjust to a new
time zone, while others experience little disruption. Crossing one or
two time zones does not typically cause jet lag.
The condition is not linked to the length of flight, but to the
trans-meridian (west–east) distance traveled. A ten-hour flight from
Europe to southern Africa does not cause jet lag, as travel is primarily
north–south. A five-hour flight from the east to the west coast of the
United States may well result in jet lag.
Crossing the
International Date Line does not contribute to jet lag, as the guide
for calculating jet lag is the number of time zones crossed, and the
maximum possible disruption is plus or minus 12 hours. If the time
difference between two locations is greater than 12 hours, subtract that
number from 24. Note, for example, that the time zone
GMT+14 will be at the same time of day as
GMT−10, though the former is one day ahead of the latter.
Symptoms
The symptoms of jet lag can be quite varied, depending on the amount
of time zone alteration, time of day and the susceptibility of
individual differences.
Sleep disturbance occurs, with poor sleep upon arrival, sleep
disruption including trouble falling asleep (if flying east), early
awakening (if flying west) and interrupted sleep with multiple
awakenings and trouble remaining asleep.
Cognitive effects include poorer performance on mental tasks and
concentration, increased
fatigue,
headaches, and
irritability, and problems with digestion including
indigestion, changes in the frequency of
defecation and consistency of
feces and
reduced interest in and enjoyment of food. Symptoms are caused by a
circadian rhythm that is out of sync with the day-night cycle of the
destination.[2]
Jet lag has been measured with simple analogue scales but a study has
shown that these are relatively blunt for assessing all the problems
associated with jet lag. The Liverpool Jet lag Questionnaire was
developed to measure all the symptoms of jet lag at several times of
day, and this dedicated measurement tool has been used to assess jet lag
in athletes.[3]
Jet lag usually requires a change of three time zones or more to
occur, though some individuals can be affected by as little as a single
time zone or the single-hour shift of
daylight saving time.[2]
Symptoms and consequences of jet lag can be a significant area of
concern for
athletes traveling east or west to competitions as performance is
often dependent on a combination of physical and mental characteristics
that are impacted by jet lag.[4]
Travel fatigue
Travel fatigue is a concept related to jet lag with symptoms of
general fatigue, disorientation and headache. It is caused by a
disruption in routine, time spent in a cramped space with little chance
to move around, a low-oxygen environment, and
dehydration caused by limited food and dry air rather than a shift
in circadian rhythms that cause jet lag. Travel fatigue can occur
without crossing time zones, and it often disappears after a single day
accompanied by a night of high-quality sleep.[2]
Management
Light is the strongest stimulus for re-aligning a person's sleep-wake
schedule and the careful control over exposure and avoidance of bright
lights can speed adjustment to a new time zone.[4]
Melatonin is used to adjust the circadian clock but there are issues
regarding the appropriate dosage and dosage timing, in addition to the
legality of the substance in certain countries.[4]
In addition, its effects may not carry-over beyond its immediate use,
and there are questions regarding how effective it may actually be.[2]
There are also concerns regarding its use by people with the symptoms of
severe
mental illness, severe
allergies,
autoimmune diseases, immune system cancers, or
pregnant women.[5]
For athletes,
anti-doping agencies may also prohibit or limit its use.[4]
Timing of exercise and food consumption have also been suggested, though
their applicability in humans and practicality for most travelers are
not yet certain and no firm guidelines exist.[4][2]
An interactive calculator that prescribes a regimen of bright light and
melatonin is available
online. There is very little data supporting the use of diet to
adjust to jet lag.[2]
While there is data supporting the use of exercise, it is hard to
determine if the exercise is responsible, or the accompanying exposure
to sunlight, and the intensity of exercise that may be required is
significant, and possibly difficult to maintain for non-athletes.[2]
Use of these strategies can occur both before departure and after
landing. Individuals may also differ in their susceptibility to jet lag
and ability to adjust to new sleep-wake schedules more quickly.[4]
Short-acting sleep medications can be used to improve sleep quality
and timing, and stimulants can be used to promote wakefulness, though
both these interventions are not generally used in non-military
situations and research results on their success at adapting to jet lag
are inconsistent. Among the stimulants, only
caffeine may be readily available to the general public.[2]
For time changes of less than three hours, jet lag is unlikely to be
a concern, and if travel is for short periods (three days or less)
retaining a "home schedule" may be better for most people.[2]
Sleeping on the plane is only advised if it is the destination's normal
sleep time.[2]
Direction of
travel
North-south flights that do not cross
time
zones do not cause jet lag. Adjustment to the new time zone is
easier for east-to-west travel than west-to-east. Adjusting to the new
time zone takes, in days, approximately two-thirds the number of time
zones crossed for eastward travel. A westward flight takes, in days,
approximately half the number of time zones crossed.[2]