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A growing body of evidence suggests the
modern world's move away from 9-to-5 jobs is taking a toll
on workers' health -- and society's pocketbook.
Shift work appears to throw
off the body's natural rhythm enough to lead to
chronic sleep disturbances, gastrointestinal problems and
even heart disease. With increasing economic and social demands,
we are rapidly evolving into a 24-hour society.
Working on non-traditional schedules,
which may include staying up all night, throws off the body's
circadian rhythms. These rhythms are governed by the body's
internal "clock" and help control the sleep/wake
cycle as well as a host of biological processes such as hormone
production and blood pressure. And the
human clock has evolved to match the light/dark cycle.
Attempts to sleep at inappropriate phases
of the circadian cycle will usually result in shorter sleep
episodes and more awakenings. Such attempts are frequent in
workers on night shifts.
The investigators compare the
short-term effects of shift work to symptoms of jet lag,
such as daytime sleepiness, disturbed sleep, gastrointestinal
problems and blunted alertness. The difference, the authors
note, is that travelers will eventually adapt to their environment,
while shift workers live out of synch with their daily surroundings.
And over time this may take a toll. A
number of studies indicate shift workers face a higher risk
of heart disease -- possibly due to the metabolic effects
of working and sleeping unusual hours.
There is also a price for society. For
one, sleep loss may make shift workers less productive. And
accidents that stem from sleepiness, such as car accidents,
exact a high cost. According to the researchers, "sleepiness-related
accidents" cost the US about $16 billion a year.
Employers and individuals need to be
aware of the major performance and alertness decrements associated
with night activity and how to best manage and counteract
them.
Some tactics that may help circadian
rhythms adapt to unusual hours include getting an adequate
amount of sleep -- whatever time of day that is -- controlling
caffeine and alcohol intake, and sleeping in a dark, quiet
environment.
Biological time is not only scientifically
important, but it also greatly
affects the productivity and health of a nation.
The cost to the nation's health of working out of phase with
our biological clocks is probably incalculable at present.
This is such an important topic that
I felt a condensed version of the article would be helpful:
The 24-h society is an environmental
challenge that outstrips our biological adaptation to the
natural 24-h cycle of light and darkness. In the course
of evolution, the behavior and physiology of most organisms,
including human beings, have developed internal temporal
characteristics.
It is thought that by timing behaviors
such as sleep so that they complement the organism's spatial
ecological niche, internal stability is maintained and the
chances of an organism's survival are increased.
In addition to health problems there is
a substantial cost to the economy in terms of decreased efficiency
and productivity. The cost of sleepiness-related accidents
can vary substantially, but in general, the estimated total
cost of such accidents per year is US$16 billion in the USA,
and US$80 billion worldwide.
Circadian (about 24 h) rhythms, are controlled
by a master biological clock. In mammals, the master biological
clock is located in the suprachiasmatic nuclei of the hypothalamus.
At the subcellular level of organization,
circadian rhythms are generated by transcriptional and translational
feedback loops involving multiple clock genes.13 The precise
periodicity (or cycle length) of the biological clock is known
to be genetically determined,14 and variation in clock genes
is thought to be related to individual differences in natural
wake and sleep times.
The biological clock generates and maintains
circadian rhythms in most physiological, biochemical, and
behavioral variables, for example:
- core body
temperature
- triacylglycerol
- blood pressure
- sleep-wakefulness
- alertness
- mental
performance
It is also responsible for the synthesis
and secretion of many hormones including
- growth
hormone
- cortisol
- prolactin
- melatonin
Melatonin
A reliable and extensively researched
marker of biological-clock activity is the rhythm of melatonin.
Melatonin is the principal hormone of the pineal gland. It
is synthesized and secreted at night in both day-active and
night-active species, thereby acting as a
signal for the length of day and night.
In human beings, sleep is normally initiated
during the rising phase of the melatonin rhythm and declining
phase of the body temperature rhythm.
Attempts to sleep at inappropriate phases
of the circadian cycle, for example during the declining phase
of melatonin and rising phase of body temperature, will usually
result in shorter sleep episodes and more awakenings. Such
attempts are frequent in workers on night shifts.
Light is the major synchronizing agent
for mammalian circadian rhythms.
Results of studies have shown that exposure
to even low light levels (100 lux), similar to that found
in offices and living rooms, will substantially affect the
phase of human circadian rhythms. However, without scheduled
activities and sleep, such intensities seem incapable of maintaining
optimum synchronization to the 24-h day.
Responses to light depend on the time
of exposure in relation to the internal biological clock:
exposure to light just after the body temperature minimum
will advance the phase of circadian rhythms, whereas exposure
before the body temperature minimum will induce delays.
Core body temperature is usually at a
minimum around 4 to 6 AM, but it can be substantially displaced
by shiftwork, jet-lag, and other situations.
In continuous darkness or in dim domestic
intensity light and in the absence of other important time
cues such as an imposed sleep-work schedule, human rhythms
free run, or become desynchronized
from the 24-h day and express the underlying periodicity of
the biological clock.
This is often seen in blind people who
have no conscious light perception. Rhythms can be synchronized
by weak time cues, but have an abnormal phase relation with
the environment.
An example is the tendency to oversleep
in winter (dim light), which in polar regions (especially
in individuals with no behavioral impositions such as scheduled
sleep wakefulness and work times) can become an overt free
run.
For those working indoors during a normal
day (0800Ð1700 h), bright natural early morning light
is only seen in the summer in the higher latitudes of temperate
or polar regions, and this
early morning light exposure might well result in earlier
circadian phase.
Timed exercise can also shift the human
biological clock, however, to date mainly phase delays have
been shown. Appropriately timed administration of melatonin
can, in addition to inducing sleepiness, phase shift and synchronize
the human circadian system.
In countries where melatonin is freely
available, it is extensively, indiscriminately, and no doubt
often inappropriately, used as a treatment for circadian rhythm
disorders and as a sleeping pill.
Shiftwork
and Jetlag
A key characteristic of the biological
clock is its ability to re-adjust (either by phase advancing
or delaying) to changes in the environment. On average, the
clock shifts about 1 h per day in the absence of countermeasures.
Symptoms of jetlag are thought to be caused
by desynchronization of circadian rhythms from the external
environment, the transient change in the phase relationship
of individual rhythms, and perhaps changes in the amplitude
of rhythms.
About Two-Thirds
Of Travelers Report Having Jetlag.
Symptoms of jet-lag include:
- daytime tiredness
- difficulty initiating sleep at night
(after eastward flight)
- early awakening (after westward flight)
- disturbed
night-time sleep
- impaired daytime alertness and performance
- gastrointestinal problems
- loss of
appetite
- and inappropriate
timing of defecation and urination
Such symptoms can seriously impair a person's
performance and ability to function, in part because of the
reduction in sleep quality and quantity, and because performance
and alertness rhythms will take several days to resynchronize.
In the long-term (eg, after 4 years),
chronic disruption of circadian rhythms from regular transmeridian
travel might result in cognitive deficits (decreased short-term
memory, slower reaction time) and changed physiological parameters
(such as cortisol concentrations).
Because of their rapidly changing and
conflicting light-dark exposure and activity-rest behavior,
shiftworkers can have symptoms
similar to those of jetlag.
Although travelers normally adapt to the
new time zone, shift-workers usually live out of phase with
local time cues.
Shift-work schedules are generally classified
in terms of the speed (rapid or slow) and direction (forward
or backward) of rotation. The issue of which schedules are
preferable from the perspective of sleep and biological rhythm
research is contentious.
On the one hand, in rapidly rotating schedules,
which incidentally are rarely used in North America, the biological
clock maintains a normal phase and workers are thus able to
continue their conventional activities during off-duty days
without symptoms of internal desynchrony.
However, the problem with such schedules
is that shifts can, and
often do, coincide with the time of day when the biological
drive for sleepiness is high and performance is low.
By contrast, a slow rotation schedule
is conducive to circadian adaptation. During
days off duty, workers typically revert to the conventional
day-active pattern. In Antarctica and in one North Sea oil
rig shift schedule complete adaptation is found, but such
situations are rare.
In the offshore situation, many more complications
are seen in sleep and performance in the rollover shift than
with 2 weeks of night shift.
The theoretical notion of directional
asymmetry in circadian adaptation to rotating shift schedules
is based on the same principles as for time zone travel; forward
(clockwise) shift rotation would result in more rapid adaptation
than backward rotation.
In addition to disruption of sleep, abrupt
changes in time cues might have negative effects on other
physiological systems. Compared with the effects of sleep,
few studies have examined the effects of shiftwork on cardiovascular,
digestive, immune, and reproductive systems, all of which
are rhythmic in nature.
Epidemiological studies are problematic;
we know that people who are intolerant to shiftwork tend to
select themselves out of such occupations.
A review of studies that investigated
shift work and risk of cardiovascular disease claimed that
on balance, shift-workers have a 40% increase in risk.
Glucose tolerance is also known to deteriorate
in the evening, and there is evidence that increased peripheral
insulin resistance might contribute to this effect. Resistance
to insulin is a putative risk factor for cardiovascular disease
and type 2 diabetes mellitus, and again, this could explain
the raised incidence of disease among shiftworkers.
Strategies have been developed to enhance
circadian adaptation to shift-work schedules and time zone
changes. Factors that promote sleep hygiene are advised, such
as:
- adequate
sleep
- sleep in
a quiet and dark environment
- control
of the use of caffeine and alcohol
- timing
sleep (with or without the
use of hypnotic agents) to the desired sleep time relative
to the new time zone or shift schedule
As described earlier, exposure to light
can phase shift circadian rhythms. Therefore, scheduled bright
light exposure and avoidance of light (possibly by use of
dark goggles) might be useful in accelerating adaptation.
Most field studies and laboratory-simulated
phase-shift studies report that correctly timed administration
of the hormone melatonin is also able to moderately shorten
the time taken for circadian adaptation.
However, there is little evidence for
optimum dose or formulation, and there is no information on
long-term safety. Further research is needed to examine how
combined administration of bright light and melatonin could
be used to develop effective, reliable, and practical treatment
strategies.
It is not always desirable to adapt the
circadian system to new shift schedules, for example in rapidly
rotating shifts, because sleep and activity on rest days will
be compromised. Similarly, when travel to a new time zone
is for a short time (eg, 1 or 2 days), circadian re-adaptation
might not be worthwhile.
In such cases, short-term strategies
can be used to maintain alertness and performance, especially
during early morning hours, and to improve sleep, without
shifting the biological clock.
Sleep Loss
and Sleepiness
Sleep loss
is obviously the most important immediate consequence of night
work.
In general, sleep loss will result in
performance deficits, including increased variability in performance,
slowed physical and mental reaction time, increased errors,
decreased vigilance, impaired memory, and reduced motivation
and laxity.
There is no consensus on the extent of
impairment resulting from a given amount of sleep loss.
Generally, complex performance tasks seem
to be more sensitive to the effects of sleep loss than simpler
tasks. It is of interest to note that the legal blood alcohol
concentration limit for driving in the UK, USA, and Canada
is 0.08%, in Australia is 0.05%, and in Sweden is 0.02%.
The decrements in performance recorded
after extended wakefulness have important implications for
shiftwork, since a substantial number of shiftworkers are
reported to be awake for at least 24 h on the first night
shift in a roster.
In reality, the temporal pattern of alertness
and performance is thought to be the result of an interaction
between circadian and homoeostatic influences. The homoeostatic
aspect, also referred to as sleep debt or sleep pressure,
will increase as a function of the duration of wakefulness
and dissipate during a subsequent sleep episode.
Models have been developed to predict
alertness levels as a function of these two factors. Such
findings can be usefully applied to shiftworkers to determine
optimum sleep-wake schedules which keep
alertness and performance at a maximum during the shift.
An important issue associated with napping
is sleep inertia, which is the feeling of disorientation and
performance impairment that happens after awakening. Estimates
of the duration of sleep inertia vary substantially, ranging
from 1 min to 4 h. Generally, sleep inertia seems to be worse
when the individual is awoken during deep, slow-wave sleep,
and after previous sleep loss.
The
Lancet September 22, 2001;358:999-1005
Correspondence to: Professor
Josephine Arendt
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