**2. What is Jet Lag?**

#### **2.1. Definition**

The American Academy for Sleep Medicine (AASM), in the second edition of its International Classification of Sleep Disorders [1] defined Jet Lag (circadian disorder, Jet Lag type), as a disorder "related to a temporal mismatch between the timing of the sleep-wake cycle generated by the endogenous circadian clock produced by a rapid change in time zones." In other words, it results from crossing time zones too rapidly for the circadian clock to keep pace.

#### **2.2. Sleep propensity in humans: circadian and homeostatic**

#### *2.2.1. Circadian influences*

In mammals, periods where sleep is favored alternate with periods where it is not. The timing, the duration and the frequency of these periods are regulated in part by the circadian body clock (see below). Timing, duration and frequency also vary across species.

Mice for instance sleep in between 40 and 120 bouts of a few minutes per day-night periods (polyphasic sleep). Human infants also show polyphasic sleep, as infants frequently wake up at night and sleep partly during the day. Most of the time, sleep episodes progressively converge to monophasic sleep with growing age, until one large sleep episode occurs at night. This monophasic organization of sleep is somewhat challenged in older life and there may be a return to forms of polyphasic sleep.

Sleep propensity in humans is maximal at the beginning of the night, moderate in the afternoon, and minimal during the morning and the evening. In parts of the world where working is difficult in the afternoon for climatic reasons, sleep has been observed to come in two phases (biphasic sleep). As our distant ancestors came from tropical areas where work is harder when the sun is high, this biphasic organization of sleep may well have been the first one to have appeared, which might explain a biphasic design.

#### *2.2.2. Homeostatic influences*

Its main symptoms include sleepiness at odd times during the day, and insomnia at night. They vary according to several variables, from individual sensitivity to age, to flight direction

Jet Lag treatment remains difficult, as no simple remedy is available, yet. As long as the earth keeps spinning round and our genome is not modified, the bad news is that Jet Lag is here to stay. Probably, some day it will be possible to turn the body clock forward or back and reset it immediately to the new time zone, but such a procedure has yet to be discovered. The natural

In this chapter, we review the scientific literature on Jet Lag origin, describe its symptoms in

The American Academy for Sleep Medicine (AASM), in the second edition of its International Classification of Sleep Disorders [1] defined Jet Lag (circadian disorder, Jet Lag type), as a disorder "related to a temporal mismatch between the timing of the sleep-wake cycle generated by the endogenous circadian clock produced by a rapid change in time zones." In other words, it results from crossing time zones too rapidly for the circadian clock to

In mammals, periods where sleep is favored alternate with periods where it is not. The timing, the duration and the frequency of these periods are regulated in part by the circadian

Mice for instance sleep in between 40 and 120 bouts of a few minutes per day-night periods (polyphasic sleep). Human infants also show polyphasic sleep, as infants frequently wake up at night and sleep partly during the day. Most of the time, sleep episodes progressively converge to monophasic sleep with growing age, until one large sleep episode occurs at night. This monophasic organization of sleep is somewhat challenged in older life and there may be

Sleep propensity in humans is maximal at the beginning of the night, moderate in the afternoon, and minimal during the morning and the evening. In parts of the world where working is difficult in the afternoon for climatic reasons, sleep has been observed to come in two phases (biphasic sleep). As our distant ancestors came from tropical areas where work is harder when the sun is high, this biphasic organization of sleep may well have been the first

body clock (see below). Timing, duration and frequency also vary across species.

(East or West), to number of meridians crossed, to length of stay and to seasons.

process of adjustment to new time zones is efficient but slow.

**2.2. Sleep propensity in humans: circadian and homeostatic**

one to have appeared, which might explain a biphasic design.

more detail, and see what can be done about it.

60 Circadian Rhythm - Cellular and Molecular Mechanisms

**2. What is Jet Lag?**

*2.2.1. Circadian influences*

a return to forms of polyphasic sleep.

**2.1. Definition**

keep pace.

The propensity to sleep is also a function of the time lapse since the last sleeping episode. The longer the time since the last sleeping episode, the stronger the pressure to enter sleep.

#### *2.2.3. Result of two processes*

Propensity to sleep is thus the result of two processes, one circadian and one homeostatic. The combination of these two processes has been first modelized by Borbély [2]. The relative weight of the two processes against each other has not been determined to the day, but I assume that they are roughly equivalent.

The intensity of both processes (circadian and homeostatic) is minimal in the morning after a normal episode of sleep, which favors awakening. As the homeostatic pressure has already decreased and the circadian influence allows for some sleep, a tendency to nap may be observed in the afternoon. The processes are at odds in the evening—the homeostasis favoring sleep but the circadian factor opposing it—which explains why it is difficult to enter sleep at that time of the day. The processes converge at the beginning of the night, hence favorizing sleep.

#### **2.3. Body clock (inner clock, internal clock, circadian clock)**

Like other mammals, human beings have a circadian clock, a paired group of nuclei in the base of the hypothalamus—the suprachiasmatic nuclei. They receive information from the retina about external light. These data allow for a daily-based correction and synchronization with the daily cycle of light and dark. The suprachiasmatic nuclei are considered together to represent the "master clock" of the brain, synchronizing a series of hormones (such as melatonin, cortisol and growth hormone), behaviors and other more specialized body clocks (core body temperature, rapid eye movement sleep, for instance).

The actions of the suprachiasmatic nuclei monitor the change from day to night. They have a useful stabilizing function on the body's biological rhythms, but they are slow to react to changes in lighting schedules, so that after time zones have been crossed rapidly, the endogenous signals for sleep and wakefulness do not match the local light-dark and social schedules.

How is the body clock indexed by external time? The major factor is the alternation of dark (night) and light (day) periods. The light signals are transmitted to the hypothalamus and support alignment of the inner body clock to the external time. There are also minor time clues (often called by their original German name, "Zeitgebers"). Minor Zeitgebers include social schedules, eating and drinking patterns, external temperature and physical activity.

Core body temperature is one of the best markers of the body clock. It is important as it has been linked with sleep and its rapid eye movement (REM) and non-rapid eye movement (NREM) components. "The ease 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. Sleep is easiest to initiate when core temperature is falling rapidly or is at its lowest and most difficult when body temperature is rising rapidly or is high. Waking is the opposite of sleep initiation, because it occurs when core temperature is rising or is high. Sleep is favored when body temperature is low or rapidly descending, as at the end of a normal evening" [3].

*2.4.5. Sleep debt*

the adaptation process.

be negatively impacted.

*2.4.6. Sleep inertia*

properly either.

**3. Jet Lag symptoms**

**3.1. Descriptive data**

An important factor in the understanding of Jet Lag is the issue of sleep debt (cumulative sleep loss). Insomnia of an hour or two during the first night will add to 1 or 2 h on the second night, and so on. The result after 2–3 days is increased sleepiness, which will favor sleep at times where a fast adaptation would indicate to stay awake (long naps at inappropriate times for instance, staying asleep too long in the morning or going to bed too early). This will delay

Jet Lag

63

http://dx.doi.org/10.5772/intechopen.75929

One hour of sleep debt has been compared to the absorption of at least 33cl of beer [5]. A cumulative debt of 4 h should thus be compared to the absorption of four-five times 33cl. Sleep debt has also been found to parallel alcohol intoxication [6]. It is easy to understand that driving a car, for instance, should be avoided. Similarly, other important activities will

And then there is sleep inertia. Coming out of sleep is not comparable to an electrical switch, it is a biological process that has its complex rules. If you have slept enough and have no circadian problem, then you should feel fully awake after about 15–30 min. But if you have a sleep debt, or are suffering from the effects of shift work or Jet Lag, or have just had too long a nap, your brain may be half asleep, sometimes for hours, unable to sleep but unable to work

Jet Lag symptoms are frequent. Rogers and Reilly [7] indicated that 74% of surveyed travelers reported some form of Jet Lag. Of these respondents, 50% reported above-average tiredness and fatigue and 28% indicated some disruption in normal sleeping patterns. And 5% reported difficulties in concentration, while 5% reported eating problems. Gisquet [8] reported 80% of sleep disorders among travelers: difficulty in falling asleep, insomnia, premature awakening and daytime sleepiness. Among very frequent travelers such as flight personnel and interna-

The most obvious Jet Lag symptoms for the majority of travelers are the inability to sleep during destination night and to remain alert during destination day. It also includes difficulty in concentrating, clumsiness, difficulty with memory, general weakness, dizziness and lethargy [10]. General malaise, dysphoric mood, headaches and gastrointestinal disorders (indigestion, loss of appetite, bowel irregularities) have also been described but cause less harm.

Symptoms logically vary according to the time of day where assessments are made. They will be lighter when day-time overlaps with a day-time at home (or when night-time overlaps with night-time at home). They will be worse when day-time is superimposed on night-time at home (sleepiness) or when night-time is superimposed on day-time at home (insomnia).

tional business executives, the disorder may be recurrent or even chronic [9].

Some of the biological cycles adjust in a few days, while others, such as the core body temperature, take more time [4].

#### **2.4. When the body clock is not in phase with external time**

#### *2.4.1. Body clock resilience: shift work*

One fundamental aspect of the circadian influence is its resilience over time. A good example is shift work. If someone with a regular work schedule abruptly shifts to a night work, that person will usually find it quite difficult to enter sleep in the morning.

This would not make sense if there was only a homeostatic force at play. Indeed, the homeostatic pressure will be maximal after a night with no sleep, since the last sleep episode ended 24 h earlier. The homeostasis hence pushes to enter sleep as soon as circumstances will allow for it.

Sleep will however be difficult to find because the body clock insists that it is time to wake up. As a consequence, sleep, if it happens, will be shallowed and fragmented. It is only after several days that the new schedule will be integrated.

And then it could be time to change the work timetable again. Continuous shift work is an unending, impossible to stabilize, and most likely unhealthy process that modern day society is unfortunately not ready to reduce.

#### *2.4.2. Body clock resilience: Jet Lag*

Jet Lag is roughly the equivalent of shift work when people are transported in a fast way (jet airplanes) across one or several meridians, that is, east- or westwards. No Jet Lag should be expected in purely north- or southwards trips. When meridians are traversed, the internal clock remains indexed, for a few days at least, to the old schedule. It is thus not adjusted yet to the new timetable.

#### *2.4.3. Shift work, Jet Lag and Zeitgebers*

There is one substantial advantage of Jet Lag over shift work. In shift work, everything around the worker reminds her that it is presently night and that other people sleep. Most shops are closed, the sky is dark, there are less sounds from the city. The time clues are working against her and do not help synchronize the biological clock. In Jet Lag, what seems strange is the daytime sleepiness and the insomnia at night, although it is basically the same phenomenon. That is because the Zeitgebers here continuously act in favor of adjustment: external light, eating and other social schedules help adjust as fast as possible.

#### *2.4.4. Jet Lag and travel fatigue*

Jet Lag must be distinguished from travel fatigue, that happens every time transport has been long and uncomfortable, be it in car, train, boat or jet. Travel fatigue vanishes after a good night sleep. Jet Lag does not.

#### *2.4.5. Sleep debt*

difficult when body temperature is rising rapidly or is high. Waking is the opposite of sleep initiation, because it occurs when core temperature is rising or is high. Sleep is favored when body temperature is low or rapidly descending, as at the end of a normal evening" [3].

Some of the biological cycles adjust in a few days, while others, such as the core body tem-

One fundamental aspect of the circadian influence is its resilience over time. A good example is shift work. If someone with a regular work schedule abruptly shifts to a night work, that

This would not make sense if there was only a homeostatic force at play. Indeed, the homeostatic pressure will be maximal after a night with no sleep, since the last sleep episode ended 24 h earlier. The homeostasis hence pushes to enter sleep as soon as circumstances will allow for it. Sleep will however be difficult to find because the body clock insists that it is time to wake up. As a consequence, sleep, if it happens, will be shallowed and fragmented. It is only after

And then it could be time to change the work timetable again. Continuous shift work is an unending, impossible to stabilize, and most likely unhealthy process that modern day society

Jet Lag is roughly the equivalent of shift work when people are transported in a fast way (jet airplanes) across one or several meridians, that is, east- or westwards. No Jet Lag should be expected in purely north- or southwards trips. When meridians are traversed, the internal clock remains indexed, for a few days at least, to the old schedule. It is thus not adjusted yet to the new timetable.

There is one substantial advantage of Jet Lag over shift work. In shift work, everything around the worker reminds her that it is presently night and that other people sleep. Most shops are closed, the sky is dark, there are less sounds from the city. The time clues are working against her and do not help synchronize the biological clock. In Jet Lag, what seems strange is the daytime sleepiness and the insomnia at night, although it is basically the same phenomenon. That is because the Zeitgebers here continuously act in favor of adjustment: external light,

Jet Lag must be distinguished from travel fatigue, that happens every time transport has been long and uncomfortable, be it in car, train, boat or jet. Travel fatigue vanishes after a good

perature, take more time [4].

*2.4.1. Body clock resilience: shift work*

62 Circadian Rhythm - Cellular and Molecular Mechanisms

is unfortunately not ready to reduce.

*2.4.3. Shift work, Jet Lag and Zeitgebers*

*2.4.4. Jet Lag and travel fatigue*

night sleep. Jet Lag does not.

*2.4.2. Body clock resilience: Jet Lag*

**2.4. When the body clock is not in phase with external time**

several days that the new schedule will be integrated.

eating and other social schedules help adjust as fast as possible.

person will usually find it quite difficult to enter sleep in the morning.

An important factor in the understanding of Jet Lag is the issue of sleep debt (cumulative sleep loss). Insomnia of an hour or two during the first night will add to 1 or 2 h on the second night, and so on. The result after 2–3 days is increased sleepiness, which will favor sleep at times where a fast adaptation would indicate to stay awake (long naps at inappropriate times for instance, staying asleep too long in the morning or going to bed too early). This will delay the adaptation process.

One hour of sleep debt has been compared to the absorption of at least 33cl of beer [5]. A cumulative debt of 4 h should thus be compared to the absorption of four-five times 33cl. Sleep debt has also been found to parallel alcohol intoxication [6]. It is easy to understand that driving a car, for instance, should be avoided. Similarly, other important activities will be negatively impacted.

#### *2.4.6. Sleep inertia*

And then there is sleep inertia. Coming out of sleep is not comparable to an electrical switch, it is a biological process that has its complex rules. If you have slept enough and have no circadian problem, then you should feel fully awake after about 15–30 min. But if you have a sleep debt, or are suffering from the effects of shift work or Jet Lag, or have just had too long a nap, your brain may be half asleep, sometimes for hours, unable to sleep but unable to work properly either.
