Seminar
Jet lag: trends and coping strategies
Jim Waterhou, Thomas Reilly, Greg Atkinson, Ben Edwards.
The number of travellers undertaking long-distance fl ights has continued to increa. Such fl ights are associated
with travel fatigue and jet lag, the symptoms of which are considered here, along with their similarities, diff erences,
and caus. Diffi culties with jet lag becau of sleep loss and decread performance are emphasid. Since jet lag
is caud mainly by inappropriate timing of the body clock in the new time zone, the pertinent properties of the
body clock are outlined, with a description of how the body clock can be adjusted. The methods, both pharmacological
and behavioural, that have been ud to alleviate the negative results of time-zone transitions, are reviewed. The
results form the rationale for advice to travellers fl ying in diff erent directions and crossing veral tim e zones.
Finally, there is an account of the main problems that remain unresolved.
Eff ect of long-haul fl ights
“I do not suff er from jet lag, only with diffi culties in
sleeping”
(Comment from an Olympic athlete after fl ying from UK to
Australia) Long-haul fl ights are associated with negative feelings after arrival that constitute travel fatigue.2–4 Panel 1 shows the main symptoms and caus. The eff ects are due to time spent in an environment that is cramped and off ers little opportunity for exerci, a restricted choice of food, dehydration due to dry cabin air,7 and cabin hypoxia, which increas fatigue and changes the daily
profiles of some variables.8 Concern has been expresd about the possibility of an incread frequency of deep-vein thrombosis in the circum-stances.9 Travel between hemispheres produces dis-orientation becau of changes in climate and natural lighting.
Many of the symptoms appear also during long road journeys. Travel fatigue disappears once travellers have ttled down at their destination. Panel 2 summaris advice on how to cope with the diffi culties associated with travel fatigue.4,10
When veral (about three or more) time zones are crosd, jet lag is noticed. The subjective symptoms of jet lag (panel 1) are similar to travel fatigue, but there are important diff erences. First, the symptoms do not disappear after a good night’s sleep—indeed, sleep diffi culties are one of the main symptoms of jet lag—but take a number of days equal to about two-thirds of the number of time zones that have been crosd. Second, jet lag is noted after time-zone transitions in the laboratory, when eff ects caud by travelling and changes in culture and meals are abnt.
Jet lag is due to the eff ects in the new time zone of an unadjusted body clock. This abnce of rapid adjustment results in loss of sleep at night, and all the daily (circadian) rhythms that are controlled by the body clock are inappropriately phad. J et lag abates as the body clock adjusts to the new time zone, a topic that has been much reviewed.11–14
The verity of jet lag increas with the number of time zones crosd, is wor for older travellers (although the reason for this fi nding is unclear),15 and
depends on the direction of the time-zone
transition—fl ights to the east are associated with more
jet lag than fl ights west.16 Sleep and circadian rhythms
are also disrupted in aircrew,17–19 thus, experience of
time-zone transitions does not act as a protection,the who
although many aircrew members change their sleep
behaviour to keep jet-lag diffi culties to a minimum.
Role of the body clock
To understand and cope with jet lag, we should be
aware of the basic properties of the body clock, and the
roles of this structure in healthy people. The
suprachiasmatic nuclei, paired groups of cells either
side of the midline at the ba of the hypothalamus, are
the site of the body clock. The nuclei have melatonin
type 2 receptors20 and receive information about light
from the eyes via the retinohypothalamic tract. There is
also an input via the intergeniculate leafl et, which is
believed to carry information about physical activity
and general excitement.
Molecular studies show that the clock is formed by
cyclic interactions between clock genes and clock
proteins, and most genetic information for the clock is
Lancet 2007; 369: 1117–29
Rearch Institute for Sport
英孚英语价格and Exerci Sciences,
Liverpool John Moores
University, Henry Cotton
Campus, Liverpool L3 2ET, UK
(Prof J Waterhou DSc,
Prof T Reilly DSc,
Prof G Atkinson PhD,
浣溪沙欧阳修B Edwards PhD)
Correspondence to:
Prof Jim Waterhou
waterhouathome@hotmail.
com
Search strategy and lection criteria
Web of Science and PubMed were arched for articles
published from January, 1997, to March, 2006. Search
phras included “jet lag”, “time-zone transitions”, and their
synonyms. Furthermore, “zeitgebers”, “light”, “exerci”,
“melatonin”, and “meals” were explored, with the limiter
“humans”. Key authors and authors commonly cited in
bibliographies were ud as further sources. More than
500 articles were obtained, but the restriction in the number
of allowed references meant that only articles thought to be
most important and methodologically sound were lected.
If veral articles had been produced by the same group of
authors, only the most recent was included. The topic of jet
lag was reviewed in this journal in 1997.1 The material
covered in that review has not been repeated, but updates
about new publications and changing emphas are
provided, as well as advice for travellers.
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conrved between species.21 In rodents studied in bright rather than dim light, the cyclic process and behaviour are disrupted by shifts of the external light-dark cycle, as would take place after a time-zone transition.22 The behavioural changes are reminiscent of jet lag in travellers. Light has also been shown to alter clock-gene expression in people.23
In the abnce of external rhythmic inputs and time cues, the body clock and daily rhythms continue, but with a period that is not exactly equal to 24 h (hence the term circadian). This period is regarded as intrinsic to the body clock. Estimates of this period have decread over the years from initial estimates of about 25 h, becau the effects of light have been progressively removed. Laboratory experiments,24 with blind or sighted individuals expod to low levels of light (150 lux or less), indicate an intrinsic period of about 24·5 h.
Whatever the exact period is for the body clock, for it to be of value its timing needs to be adjusted to the solar day. This adjustment is brought about by rhythmic cues in the environment, known as zeitgebers (time-givers). The effect of a zeitgeber on the body clock depends on its time of prentation; a zeitgeber can produce a pha advance, pha delay, or no pha shift. This relation between the time of prentation of the zeitgeber and the pha shift produced is called a pha-respon curve. The main zeitgebers are the light-dark cycle and the rhythmic cretion of the pineal
hormone melatonin (taking place during nocturnal sleep in healthy people); however, exerci
Panel 1: Common symptoms of travel fatigue and jet lag, and differences in their caus
Travel fatigue
Symptoms
• General fatigue.
• Disorientation and incread likelihood of headache.
• Travel weariness.
Caus
• Disruption of sleep and normal routine, diffi culties associated with travel (checking in, baggage claim, customs clearance), and general dehydration.
Jet lag
Symptoms
• Poor sleep during the new night-time, including delayed sleep ont (after eastward fl ights), early awakening (after westward fl ights), and fractionated sleep (after fl ights in either direction).
• Poor performance during the new daytime at both physical and mental tasks.
• Negative subjective changes. The include incread fatigue, frequency of headaches and irritability, and decread ability to concentrate.
• Gastrointestinal disturbances (indigestion, the frequency of defecation, and the consistency of the stools) and decread interest in, and enjoyment of, meals.
Cau
• Slow adjustment of the body clock to the new time zone, so that daily rhythms and the internal drive for sleep and wakefulness are out of synchrony with the new environment.
Differences between travel fatigue and jet lag
• Travel fatigue is associated with any long journey; jet lag generally needs three or more time zones to be crosd rapidly.
However, there is a noticeable diff erence in individuals’ susceptibility to the eff ects of changing time; some even have diffi culty in dealing with the 1 h change accompanying the switch to and from daylight saving time.
• Travel fatigue abates by the next day, the traveller having had a good night’s sleep; jet lag after eastward fl ights lasts for veral days roughly equal to two-thirds of the number of time zones crosd,5 and about half the number of time zones crosd after westward fl ights. Again, there are obvious diff erences between individuals.6
Panel 2: Advice for coping with travel fatigue
Before the journey
Plan the journey well in advance.
Arrange for any stopover to be comfortable.
Arrange documentation, inoculations,visas, etc.
Make arrangements at the destination.
During the journey
Take some roughage (eg, apples) to eat.
Drink plenty of water or fruit juice (rather than tea, coff ee, or
alcohol).
On reaching the destination
Relax and rehydrate with non-alcoholic drinks.
Take a shower.
Take a brief nap, if needed, but not enough to stop getting to
sleep at night.
Seminar exerts a much weaker eff ect than do other zeitgebers.
Figure 1 shows simplified pha shifts that can be
produced in individuals by the zeitgebers. There is
interindividual variation in the timing of the
temperature minimum (Tmin) and dim-light melatonin
ont, and the switches from pha advance to pha
delay or no shift are less clear-cut than might be
evanstoninferred from this fi gure.26,28
The body clock brings about daily rhythms in core
temperature, plasma hormone concentrations, and the
sleep-wake cycle—all of which, in their turn, exert
widespread effects. Thus, the whole body becomes
rhythmic. The body clock is not readily perturbed by
external factors, which is uful when waking during
the night or naps during the daytime are considered,
since a change in timing of the body clock would be
inappropriate then. However, this resistance to pha
shifting is the cau of many of the diffi culties associated
with time-zone transitions.
The body clock promotes activity in the daytime and
recovery and restitution during the night, which
enables preparations to be made for the switches
between the active and sleeping phas. The circadian
rhythms of core temperature and melatonin cretion
are cloly associated with the rhythm of sleep
propensity. Superimpod on the rhythms are eff ects attributable to the sleep homoeostat. When the amount of time awake, fatigue, and the need for sleep increas, alertness falls. The changes are reverd during sleep.
The ea of getting to sleep and staying asleep depends not only on previous wake time but also on associations with the circadian rhythm of core temperature.29 Sleep is easiest to initiate when core temperature is falling rapidly or is at its lowest, and most diffi cult when body temperature is rising rapidly or is high. Waking is the opposite of sleep initiation becau it happens when core temperature is rising or is high. As a result, wakefulness is maintained in the daytime30 and, since the normal core temperature minimum occurs between 0300 h and 0700 h (fi gure 1), the conventional times of sleeping are synchronid with the rhythm of core temperature so that an unbroken night’s sleep can be obtained. As a result, when the body clock is inappropriately phad, sleep is diffi cult to initiate and maintain.
There is a general parallelism between core temperature and mental performance, but deterioration becau of time awake is also important, especially for tasks that need large amounts of central proc
essing or short-term memory. Mental performance improves with rising core temperature throughout the early hours of the waking day; in the latter half of the day, tasks requiring little central processing or memory (eg, simple reaction time) continue to be done well until the evening, although mood changes and tasks needing more central processing and short-term memory (eg, decision-making) deteriorate becau of fatigue before the evening fall in core temperature.31 Several mathematical models32 have assd changes in mood and performance under diff erent combinations of time of day and time awake, many of which include an additional deterioration in mental performance caud by sleep loss.
There is evidence4 that many aspects of physical activity display circadian rhythms that are cloly in pha with that of core body temperature. The aspects include peak force of muscle contraction, anaerobic power output, performance in long-jump and high-jump, and an individual’s motivation to undertake sustained effort. Furthermore, sports that simulate contests or that involve timing skills (eg, swimming, cycling, football, tennis, and badminton) show circadian variation.
However, the view that there are circadian rhythms in physical activity has been criticid.33 One criticism relates to the frequency of measurement during the cour of a day. Maximum physical exertion is far more diffi cult to measure repeatedly than is sleep propensity or mental performance,
becau of muscle and biochemical fatigue.34 Another challenge is that the extent to which the body clock is responsible for any obrved rhythms has often not been established, since such rhythms have a substantial exogenous component (due to sleep-wake cycle). Even so, veral variables related to sports performance do show an endogenous component (due to the body clock).35 Even though sleep loss ems to have little eff ect on muscle strength, top performance requires components—eg, mood, strategy,
Figure 1: Pha shifts (advances and delays) of the body clock produced by light, melatonin ingestion,25,26 and exerci27 at different times during the day Body clock markers, dim-light melatonin ont (DLMO), and the minimum of
core temperature (Tmin), are shown, and the shaded area shows a range of Tmin that is usually en. Horizontal black bar indicates normal sleep time.
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and the desire to train to a maximum—that are aff ected by sleep loss.36The importance of the body clock itlf, rather than the rhythms of core temperature and plasma melatonin, in the production of the rhythms in sleep and performance is unresolved; for individuals living healthily and who are adjusted to their environment, the body clock and circadian rhythms are synchronid to one another, which makes it very diffi cult to parate out their individual roles.Time-zone transitions
A recorded rhythm can be regarded as a mixture of
小衣components becau of eff ects of the body clock (the endogenous component) coupled with eff ects of the individual’s environment and lifestyle (the exogenous component). The components are usually synchronid, but this synchronicity is lost in the days after a time-zone transition, since, unlike the exogenous component, the endogenous component has not adjusted. The eff ects of this loss of synchrony can be en by comparison of hypothetical data showing two rhythms before and after a time-zone transition (fi gure 2). The rhythm with a substantial exogenous
component ems to adapt its pha immediately. However, a rhythm with a larger endogenous than exogenous component, becomes distorted after the transition, showing poor pha adjustment and a
decread amplitude. Heart rate has a large exogenous component, rectal temperature has endogenous and
exogenous components of about equal size, and the rhythm of melatonin cretion, when measured in dim light, has only a small exogenous component.In general, therefore, overt circadian rhythms will
adjust to time-zone transitions at diff erent rates. Tho with a larger exogenous component—eg, food intake 37 and physical activity—will em to adjust more rapidly
than w ill t ho w ith a l arger e ndogenous c omponent—eg, sleep, mood, and mental performance.wsbs
Poor sleep (assd by actimetry, by polysomnography, and subjectively) is one of the main drawbacks associated with time-zone transitions.38,39 After a westward fl ight, individuals feel tired during the new evening by local time and yet wake prematurely (becau of rising core temperature and falling melatonin cretion produced by their unadjusted body clock).
After an eastward fl ight, individuals do not feel tired at
midnight by local time (daytime by their unadjusted body clock); however, they are ready for sleep as the new day dawns. Like jet lag, sleep disturbances are wor after eastward fl ights than after westward fl ights.38 Mental performance, mood, and alertness all worn, due not only to the endogenous component of the rhythms (ie, being awake when body temperature is low and attempting sleep when it is high) but also becau of sleep loss.31
Several studies show that elite athletes have sleep loss
containand mood disturbance after long-haul fl
ights.4,39–41 Moreover, athletes travelling fi ve time zones to the west
showed shifts in performance profi
les (including grip, back, and leg strengths) that were in pha with the unadjusted rhythm of intra-aural temperature.42 A similar change of daily profi les in grip strength was en in a group of Olympic athletes and dentary people travelling ten time zones to the east.43 Lemmer and co-workers 44 reported that elite athletes travelling to the west or east over six to eight time zones showed
pogo
altered grip strength and poor performance in training
ssions for veral days after the fl ight.With repeated long-haul fl ights, there is evidence for some long-term diffi
culties for aircrew. Irregularities of the menstrual cycle have been known for some河北省学位考试报名网
time, and the fl
uctuations have been linked to altered patterns of melatonin cretion.45 Defi cits of
cognitive performance 46 and increas in psychotic and
major aff ective disorders 47 have also been described. Such eff ects have not been reported in healthy travellers who experience of time-zone transitions is far less extensive than tho who regularly travel long distances.
Figure 2: Diagrammatic reprentation of circadian rhythms of variables with a large endogenous or
exogenous component Upper=before time-zone transitions in control conditions, with subjects sleepi
ng 2400 h–0800 h. Lower=after time-zone transition, simulating the immediate eff ects of an 8-h time-zone transition to the east. The exogenous component was described as +1 during waking and –1 during sleep times. The endogenous component was described as a cosine curve (mean of 0 and amplitude of 1) peaking at 1700 h during control time and not
adjusting to post-shift time (ie, peaking at 0100 h by the new local time). Before the time-zone transition, the rhythms are similar in pha and amplitude. The exogenous rhythm (exogenous and endogenous components mixed in the ratio 4 to 1) ems to adjust in pha to the new local time and with little change in shape. By contrast, the endogenous rhythm (exogenous and endogenous components mixed in the ratio 1 to 2) changes in
shape and its pha adjusts very little to new local time.
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One implication of diff erences in fl exibility of habits, age, or chronotype48 (ie, whether their circadian rhythms and lifestyle are phad earlier or later than average) is that there might be diff erences in peoples’ respons to time-zone transitions. In one study,49 travellers with rigid sleeping habits had more symptoms of jet lag than did tho with less rigid sleeping habits. Another study of e
lite athletes and their trainers50 failed to show an eff ect of chronotype on the amount of jet lag; instead, the amount of jet lag depended on the individuals’ travel schedule. Despite having slept substantially less during the flights, tho who schedule had a shorter interval between their last full sleep before the flight and their first one at their destination had less jet lag than tho who schedule resulted in this interval being longer. The contribution of diff erent light exposures was not assd.
People older than 60 years have less regular circadian rhythms, and lower amplitude, pha-advanced body temperature and melatonin rhythms than do younger controls.15,30,51 The individuals also have greater diffi culty in coping with jet lag, especially after eastward fl ights.15,16 The pha-respon curve to light is slightly different in older people.52 However, agomelatine, a melatonin agonist, can promote pha advances in elderly men when given in the evening,53 and exerci taken at bedtime induces pha delays similar to tho en in younger controls.54
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Alleviation of jet lag
There is an obvious premium on promotion of body clock adjustment, which requires knowledge of the zeitgebers that adjust the body clock to its normal state. Furthermore, amelioration of the sympto
ms of jet lag—especially poor-sleep improvement and reduction of daytime fatigue—will be benefi cial, even though this improvement need not include adjustment of the body clock.
The best way to alleviate jet lag is by adjustment of the body clock. The main zeitgebers in individuals are the light-dark cycle and the nocturnal cretion of melatonin. Sleep, physical activity, and food intake have also been implicated. The eff ects of light exposure and melatonin cretion act to synchroni the body clock with the sleep-wake and light-dark cycles.24
A pha-respon curve to light (fi gure 1) has been established.55,56 Exposure to gradually increasing light intensity57 rather than a pul of light exerts a modest eff ect, and continual puls of light em to be almost as eff ective as continuous light over the same period.58–60 The amount of pha shifting depends on light intensity, and small shifts are produced by domestic lighting, which is important to tho who exposure to natural light is restricted.55,61,62
The detailed nature of the receptors, the wavelength of light they respond to most,63,64 and their distribution in the retina,65 remain uncertain. Some believe that the photonsitive pigment is derived from vitamin B2,66 but this theory is disputed.67,68 The claim69 that light could be received by non-ocular receptors (situated at the back of the knee) has not been substantiated.70–73 Change
s to the time of sleep shift the body clock in the opposite direction to that produced by light given at the same time.74–77 The role of sleep itlf (rather than lack of exposure to light) is difficult to asss since shutting the eyelids introduces a light-dark cycle. However, Danilenko and co-workers78 reported small advances of the body clock when sleep was advanced by 20 minutes daily in individuals who remained in light intensities less than 0·2 lux. Even if sleep itlf can act as a weak zeitgeber, the eff ects are small compared with tho caud by light exposure associated with a normal lifestyle.
The chemistry and roles of melatonin have been much reviewed.79–84 A pha-respon curve to melatonin shows that exogenous melatonin delays the body clock after waking and advances it in the afternoon and evening (fi gure 1).25,26,85,86 However, pha delays have proved difficult to show convincingly.87 Since light inhibits the relea of melatonin, the two zeitgebers act jointly—eg, light on morning awakening advances the body clock directly (through the light pha-respon curve) and also becau of suppression of melatonin cretion (which would cau a pha delay at this time). Exposure to bright light coupled with melatonin ingestion, with the times designed to promote pha shifts in the same direction, has produced additive eff ects.88,89 However, if the two interventions are given at the same time (when some degree of antagonism is predicted; fi gure 1), the extent to which they oppo one another is disputed.90,91
The light intensity needed to suppress melatonin cretion is lower than that once thought, especially if the individual has recently been living in low light intensities,92–94 and some argue82 that such a result needs a reasssment of the melatonin pha-respon curve. There is some evidence that melatonin cretion is suppresd more by light in the upper visual fi eld,95 and that shorter wavelengths (blue-green light) are more eff ective in causing this suppression and pha shifts of the rhythm than are longer wavelengths.63,96,97
Melatonin ingestion also has a soporifi c eff ect and lowers core temperature.98–100 Pineal cretion normally begins in the evening and is associated with the increa in sleep propensity and decrea in core temperature at this time.98,101 The outcomes might be a direct eff ect of melatonin on the suprachiasmatic nuclei or a vasodilatory action on the cutaneous vasculature via melatonin type 2 receptors, or both.20,102 Vasodilatation will lower core temperature, but fatigue and the ability to fall asleep are associated more with the ri in distal cutaneous temperature than with the fall in core temperature.103
