Air University Review, July-August 1968
Since the beginning of motor technology the mode and speed of travel have shown a revolutionary development. Before this technological time, back to the era of the caveman, man stayed in his local time zone with its regular cycle of day and night to which his activity and sleep cycle were adapted. The development of transcontinental and intercontinental surface transport vehicles within the past hundred years made it possible to travel through some four to five time zones within a week or fractions of a week. With the appearance of aircraft at the beginning of this century, a radical change took place Subsonic and supersonic speeds have enabled man to cross half a dozen time zones within a fraction of a day. An even more radical change has been achieved with the orbital velocities of rocket-propelled spacecraft, which cross half a dozen time zones within fractions of one hour, making time zones meaningless for the spaceman. But, like the caveman, the spaceman requires a regular sequence of activity, rest, and sleep. This physiological rhythm is usually synchronized with the physical day-night cycle in the local time zone of man’s habitat. Rapidly crossing the time zones, as in travel by air or moving beyond the realm of time zones as in space travel, poses problems of special medical, professional, and general human interest.
Before discussing this topic in detail, we have to familiarize ourselves with man’s natural sleep and wakefulness cycle, regulated by his physiological clock.
The considerable progress made in our knowledge of the nature of sleep and wakefulness has been elaborated in detail in the classic book, Sleep and Wakefulness, by N. Kleitman, but I must confine myself to a few essential points.
During the state of wakefulness we are aware or conscious of the outside environment and of some processes inside our body via the exteroceptors and interceptors, respectively. There are various levels of wakefulness described as alertness, attentiveness, and vigilance.
During sleep we are not aware of the outside world. There are also different stages in the depths of sleep—drowsiness as the first stage, light sleep (second stage), and deep sleep (fourth stage), with a transitional stage between the last two. Light sleep is the phase of dreams and rapid eye movements. Concerning the amount of sleep, in addition to the six to eight hours of nightly sleep many people enjoy a short afternoon nap. Furthermore, after a night with not enough sleep, sleep seizures occasionally occur, lasting only several seconds therefore called “micro-sleep”; they can be cause of auto accidents late at night. A deeper insight into the stages Or levels of sleep and wakefulness has been gained by recording the electric activity of the brain.
During the state of wakefulness the electroencephalogram shows one dominant
frequency of “brain waves” of about 9 to 13 oscillations per second and of
several millivolts—the so-called alpha waves. During
sleep their frequency decreases to
Wakefulness, characterized by mental and voluntary motoric activity, and restful sleep are the most conspicuous signs of the physiological day-night cycle, now called the circadian cycle (from circa and dies = about a day), (F. Halberg). Many more changes behind the visible scene are found in the activities of practically all other body organs. Their special functions can be recorded by electrical and biochemical methods.
During sleep, activity of the voluntary motoric muscles practically disappears except for some dozen reflex movements triggered by the so-called gravitational pressure points. The muscle tone is relaxed. Consequently, during sleep we observe a slowdown in metabolic rate, respiration, heart rate, and blood pressure.
In contrast, motoric and secretionary activity of the digestive system increases during the night; but the kidneys and urinary transport system are more active during the daytime. The blood, as a kind of mirror, reflects the picture of the overall activities in the form of day-night variations in its cellular and chemical constituents.
Of special significance is the hormone secretion of the endocrine glands
because they play an important role in control of the wake and activity cycle.
Adrenalin production by the medulla of the adrenal gland is at a maximum
between 4 and
All these periodic variations on an organ, cellular, and molecular level, harmoniously integrated into a functional circadian system with the hypothalamus as the coordinating center and the hormones as the intermediary chemical agents but strongly influenced by the cerebral cortex, repeat themselves with a clock-like regularity within the temporal frame of 24 hours. This has led to the coining of the terms “metabolic clock” and “physiologic clock,” the best-known indicator of which is the body’s temperature, which shows a peak in late afternoon and a low in early morning.
All in all, the body’s clock is actually a clock system composed of various subsystems with their own individual clock indicators. This clock system as a whole is predominantly a circadian one-wave system, but the subsystems show additionally intradian fluctuations in their activities in order to meet the needs of the whole body system at the right time.
The natural time signal (Zeitgeber—J. Aschoff) time cue or “synchronizer,” for the phase setting of sleep and wakening within the circadian cycle is the change from darkness to light at sunrise and back to darkness at sunset. This refers to both the “light active” or diurnal and the “dark active” or nocturnal creatures.
The adult’s physiological requirement for sleep is about 7 + 1 hour every 24 hours, plus one or two catnaps for those people who are not under community, social, or professional pressure. As mentioned earlier, there are various stages in the depth of sleep—light sleep and deep sleep, with transitional stages between. As a measure of sleep, the concept that the amount of sleep is the product of the duration and the depth of sleep has been suggested (W. von Frey, 1930). Most people at the beginning of their night’s sleep fall into a deep sleep for two to three hours, which is followed by a light sleep. Some people seldom enjoy a deep sleep; these light sleepers need more time than deep sleepers to get the same amount of sleep. But there is at present no unit for measuring the amount of sleep comparable to the calorie for measuring metabolic requirements. Be that as it may, within the time frame of 24 hours man needs a certain amount 0 sleep to restore the energies spent in physical and mental activities during the phase of wake-fulness.
The circadian cycle in man and numerous animals shows a certain degree of stability, which may be properly expressed by the term “cyclostasis” if “stasis” is used in the same sense as in “homeostasis” or the tendency of the body to keep the physical and chemical properties of the intercellular body fluid nearly constant (proposed by W. B. Cannon in his book, The Wisdom of the Body, 1929). If “homeostasis” means controlled internal environment, then “cyclostasis” means controlled internal rhythmicity.
The stability of the circadian cycle, or the cyclostatic nature of man, is demonstrated by the following facts:
(1) It is impossible to break this cycle by ignoring sleep completely over a
number of days; this sleep loss would lead to neurotic disorders, as proven by
numerous sleep-deprivation experiments. Also from history it is known that
Napoleon I and
(2) According to Kleitman’s sleep studies in Mammoth Cave, Kentucky, the duration of the circadian rhythm can be shortened to 18 hours or extended to 28 hours by exposing the individual to artificial light-dark cycles; the physiological clock accepts these durations by adaptation. But going below this minimum or beyond this maximum is outside the clock’s adaptability, and it continues to run at its routine optimum 24-hour cycle.
(3) The sleep and wakefulness cycle continues in its nearly circadian pattern in constant photic environments, as observed on inhabitants of the subpolar twilight zones and on animals kept under similar constant conditions in the laboratory.
(4) The physiological circadian cycle can be shifted in reference to the physical one, but it requires a number of days for readjustment. This is a familiar problem in work shifts in industries, communication and transportation services, fire and police departments, hospitals, military services, astronomical observatories, on ships, etc. Individuals involved e professional activities feel some inconvenience when they have to change the work time. There are, of course, individual differences in the sensitivity to a phase shift. Some e can sleep like a cat any time, any place, under any conditions, but the majority are more or less sensitive in this respect. This is even indicated by the public reactions to such a small time change as that to daylight saving time.
These four points definitely illustrate the basic cyclostatic nature of the human body in terms of a relative stability of its internal circadian cycle.
In all these examples of changes in work time, the individual stays in his home time zone. With the development of fast-moving ace vehicles and especially since the advent of the airplane, a new way of phase shifting of the day-night cycle is experienced by millions of people, that is, by time zone changes during travel, particularly by air.
the physiological clock in air
travel
Within the higher range of subsonic speed and in supersonic speed, about half a dozen zones are crossed in six hours or less. This exposes a traveler in a very short time to a day-night cycle different from that at the point of departure and consequently different from the physiological day-night cycle which dominates his body. This means a phase shift between the geographic and physiologic cycles. Flight in an easterly direction advances the cycle and In a westerly direction delays it.
This unaccustomed relation of the internal time of the traveler to the local geographic time is called desynchronization or desynchrony, and it may take him from several days to a week to get adapted to the local time at the termination of the trip, or until the two cycles, physical and physiological, are resynchronized.
As statistical studies in long-distance flights have shown, the majority of people are sensitive to this travel-produced phase shift and experience some discomfort for several days. They become hungry, get sleepy, or are wakeful at the wrong time with regard to the new local time. Their “head clock” and “stomach clock” and elimination system are confused. Such is, in brief, the picture of the circadian “phase shift syndrome.”
After transcontinental flights in the
It must be emphasized that the psychophysiologic effect of cycle desynchronization, or, more in line with medical language, desynchronosis, is not a pathologic condition; it is merely a time disharmony concerning what the body’s physiological internal milieu expects from the physical and social external milieu at the new locale. But this time disharmony can be significant in many respects.
First, circadian cycle desynchronization can have some significance concerning political summit meetings, emergency sessions of the United Nations, international scientific congresses, the Olympic games, etc. During the first few days of such events the participants who had to cross a number of time zones may be temporarily in a somewhat handicapped position due to their desynchronotic condition.
The problem of circadian cycle desynchrony is especially important for those whose occupation involves time zone changes. Pilots of long-distance air routes and stewardesses as well are in this category. Too frequent shift of their circadian cycle causes fatigue and requires special attention; this is well recognized and taken care of by the pilot associations, the medical directors of the airlines, and the medical officers of the Air Force.
What can be done to avoid the state of desynchronization of the circadian cycle on a certain occasion which requires full alertness after long-distance flights? There are several ways to achieve this and to be synchronized with the local time at the destination of the journey at the right time.
First, if an individual has to attend an important meeting at a distant location he can preset his physiological clock by adopting several days before the trip a sleep and wakefulness pattern corresponding to the physical day-night cycle of the place in question. This is preflight adaptation or synchronization. When traveling in an easterly direction, one can do this by going to bed every evening one hour earlier, and in a westerly direction one hour later, beginning three to five days before departure.
Second, the individual can fly to the distant place several days in advance of a certain event or meeting; this is postflight local preadaptation.
Each of these two methods, preflight and postflight preadaptation, should be effective to keep the traveler alert during the day desired.
A traveler who cannot afford the time for preadaptation should know that the morning hours during the first days after long eastbound flights and the late afternoon hours after westbound flights are not the proper times for important discussions and decisions.
Finally, mild medication, taken at the proper time, might be helpful’to accelerate, as a kind of biochemical synchronizer, the physiological adjustment to the new local time.
All this is of no concern to vacation travelers.
Furthermore, the higher-speed jets can also mitigate the problem of time zones and circadian rhythm in that they allow flight to a distant place and back in one day; for instance, from cities on the East Coast of the U.S.A to the West Coast and return. In this case phase shift is an intradian matter and should cause no desynchronization symptoms, especially if the speed is supersonic, which makes possible even a round trip from Washington, D.C. to the capital cities of Europe within on day.
the physiological clock in
space flight
Supersonic speed exceeding mach 5 is called hypersonic speed. This third aeronautical speed blends with the first astrononautical or cosmic velocity (8 km/sec) which permits orbital flight. With this we enter a completely novel situation—the customary geographic day-night cycle is replaced by a cycle of short sunlight, short earth-shadow.
Within the relatively radiation-safe altitude range from 200 to 800 km below the inner Van Allen radiation belt, the orbital last from 80 to 130 minutes. About 30 percent of this time, depending upon the orbit inclination, the spacecraft is in the earth shadow. The external photoscotic (light-shadow) period is not longer than one-tenth of the light-darkness cycle on earth.
But in this unearthly photic environment of near-earth space, with short, contrast-rich photic periodicities, orbiting astronaut arrangement of their sleep and activity regime have to follow the dictate or, better, “tick-tock” of their internal clock. It has to be isochronous with their natural inborn circadian pattern, and preferably synchronous with their home time zone. (Isochronous: occurrence in equal time periods; synchronous: occurrence at the same time.)
In addition to the absence of a suitable external light-dark cycle, the absence of weight enters the life of the astronauts. Fortunately, sleeping under weightless conditions seems to be no problem. One of the reasons: no gravitational pressure points. Furthermore, weightlessness makes the parasympatheticus dominant, which conforms with the sleep-induced parasympathicotonia. All our astronauts and the Russian cosmonauts had a sound sleep when the radio noise level was kept in proper relation to the silence of space. With two astronauts on space flight, it has been found practical for them to sleep at the same time, so that Space Center on earth both of them radio silence for sleep and still leave maximum time for communication with them (Berry). With a crew of more than two, a properly arranged shift in the sleep and activity cycle will be required.
All in all, nothing illustrates more clearly the built-in cyclostasis of the human body than space flight with its exotic photic environment.
On the moon, the physiological sleep and activity cycle will be completely independent of the physical or selengraphic day-night cycle, which is 27 terrestrial days in length. This photic environment does not provide a “time cue” comparable to earth’s sunrise and sunset within 24 hours. The selenonauts must arrange in the lunar station the sleep and activity regime that accords with their geocyclostatic nature. Fortunately, since the moon’s low gravity (one-sixth of one g) would probably cause fewer sleep-interrupting body movements than on earth because of lighter pressure points, sleep might be more refreshing in the easeful arms of Luna, goddess of the moon.
On the most attractive postlunar astronautical target, the Red Planet, Mars, the day-night cycle is only 37 minutes longer than that on earth. Thus the temporal day-night alternation on Mars offers a time sequence familiar to terrestrial visitors for their sleep-activity cycle, and consequently there should be no difficulties in this respect on a Martian station.
In conclusion, the circadian cycle of the physiological clock, as an inborn property of the human body, is and will remain of medical and professional interest in air travel with reference to the global network of time zones, and beyond them it will play a vital role in the success of man’s further conquest of space.
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Acknowledgment
The author wishes to acknowledge consultation with Colonel R. F. Fitch of
Dr. Hubertus Strughold
(Ph.D.,
Disclaimer
The conclusions and opinions expressed in this
document are those of the author cultivated in the freedom of expression,
academic environment of
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