Industrial Health 2005, 43, 24–29
*To whom correspondence should be addressed.
The Nighttime Nap Strategies for Improving Night Shift
Work in Workplace
Hidemaro TAKEYAMA1*, Tomohide KUBO1 and Toru ITANI1
1Health Sciences of Life, Work and Environment, Department of Environmental Health Science and Health
Promotion, Nagoya City University Graduate School of Medical Sciences, 1 Kawasumi, Mizuho-cho, Mizuho-
ku, Nagoya 467-8601, Japan
Received September 24, 2004 and accepted November 26, 2004
Abstract: Nighttime napping is an effective measure to prevent adverse effects due to night shift
work. A characteristic of nighttime nap is that it can result in considerably deeper sleep. Several
studies have shown that taking nighttime naps suppressed increasing sleepiness, decreasing alertness
during the period following awaking from a nap, and prevented disturbance of circadian rhythm
(“anchor sleep”). The length of daytime sleep after night shift, when combined with a nighttime
nap, is shorter than that without nap. This finding might be interpreted as a beneficial effect rather
than a negative feature because workers can then spend time engaged in other activities rather than
sleeping. Nevertheless, the introduction of nighttime sleep break in the workplace has not been
widely accepted. To promote nighttime napping strategy in the workplace, consensus building while
acting on conflicting interest is essential. Recently, participatory method for improvement of working
condition has expanded worldwide. A characteristic of the activity is using action checklist and
group work, and heightening motivation to improvement working condition between worker and
manager. Through the activities, nighttime napping strategy would be spread more in the workplaces
and play a role as one of the effective tools for improving working conditions, work performance
and safety in the future.
Key words: Nighttime nap, Napping strategy, Shift work, Safety, Health, Participatory activity
Recently, the demand for 24-h operations in various types
of industries has expanded worldwide. In multi-shift work
schedules, workers need to operate well when on duty, despite
sleeping irregular hours. Daytime sleep is often of a shorter
length and provides lower restorative values than a full night’s
sleep1). During a night shift, alertness and performance often
diminish as time progress. Night- shift work can cause not
only occupational accidents due to severe sleepiness and a
resulting reduction in alertness but also can introduce several
health problems among these non-daytime shift workers2–4).
Previously, various countermeasures to prevent adverse effects
due to night-shift work have been examined. Exposure to
stimulant environment condition such as “light”, “sound”,
and “temperature”, intake of caffeine or pharmacological
substances, and taking a nap has been investigated to enhance
and sustain worker’s alertness during night shift5). Taking a
nap particularly has known one of effective countermeasure
to prevent increasing sleepiness and decreasing alertness.
Previous nap research related to nightshifts has examined two
kinds of nap strategies to help avoid reduced the work
performance among night workers6, 7). One is a nap taken in
advance to avoid sleep deprivation (“prophylactic nap”)8, 9),
and the other is a nap taken during the night shift (nighttime
nap)10–13). Some studies have indicated that a prophylactic
THE NIGHTTIME NAP STRATEGIES FOR NIGHT SHIFT WORK
nap sustains alertness and performance during the
nightshift19, 20). However, if night-shift workers try to sleep
before their night shift, generally they have to fall asleep at
the nadir of sleepiness (“forbidden zone” for sleep)14). On
the other hand, nighttime nap is not only deer sleep but also
preventing the disturbance of circadian rhythm due to night
shift. However, the introduction of nighttime naps in the
workplace has not yet become widespread15). It seems there
are various practical problems when implementing napping
into the nighttime workplace.
In this paper, we reviewed past studies regarding nighttime
napping, and discussed the roles of nighttime naps and various
considerations when a napping strategy is implemented in
a night-shift work environment.
Expanding Nighttime Napping in the Workplace
How many workplaces have introduced nap systems, and
how long and how often workers nap are important matters.
Previous surveys, most of them conducted in Japan, have
indicated an increase in nighttime napping programs in the
workplace. In an earlier study, Matsumoto found that 38.6%
of the night-shift workers engaged in the pulp-paper and
other industries (458 firms) took nighttime naps16). Sakai
et al. conducted a questionnaire survey among 828 iron and
steel firms (continuous process plants) and found that taking
nighttime naps accounted for 37–43% of the male shift
workers and 37% of the female on night shifts17). Nishiyama
et al. conducted a questionnaire survey in the Japanese
chemical and allied industries (1,844 shift workers), and
reported that 49.5% of these shift workers take a nap during
their night duty18). Sakai and Kogi examined how long
workers could take a nap in continuous process plants in
the Tokyo area19). Most three-shift workers (8 h shift) take
2–3 h (70.4%), while other workers take 0.5–1 h (15.7%)
or 4–5 h (7.4%). In the case of two-shift (16 h shift), 50.6%
of workers take 2–3 h and 46.1% of workers take 4–5 h.
These findings indicate that nap length partially depends
on shift length. Sasaki et al.20) reported that naps were rarely
taken if the night break was less than 30 min. When a break
was allowed for 30–59 min, about one-third of the workers
took a nap. In the cases of 60–89 min and 90 min or more,
about 80% of the workers took a nap. These results indicate
that the longer the break that is allowed, the more workers
take naps. Taking a nap during the night shift has been
introduced in Japanese industries with a high percentage of
approval among workers21). However, little attention has
been given to the value of napping during night duty in most
other countries. Therefore, it can be expected that these
values are higher in Japan than those in other countries.
The surveys indicate some complaints such as “too short”,
“disturbed by noise”, “uncomfortable temperature”, and
“filthy bedclothes” from workers who take naps because
some industries are not equipped with a comfortable room
separated from the work site even though they may have
introduced a nap system17, 18). These findings indicate the
necessity of considering not only shifts designed to allow
naps but also the facilities suitable for napping.
Effects of Nighttime Napping
Advantage and disadvantage of nighttime napping
A characteristic of nighttime naps is that it can result in
considerably deeper sleep as indicated by lower body
temperature, much as Slow Wave Sleep (SWS)22). The
physiological feature of the nighttime nap during the initial
period of nighttime napping from the electroencephalographic
aspect is comparable to that of normal night sleep23). Several
studies have indicated that a nighttime nap prevented reduction
in alertness and performance, and reduced sleepiness during
the period following awaking from a nighttime nap, although
there are difference of methodology6, 10–13).
Another advantage point of the nighttime nap, “anchor
sleep”, has become known, and was introduced by Miner
and Waterhouse24). They divided an 8-h sleep into two 4-h
sleep periods. One of the 4-h sleep periods was taken
consistently at the same time (0:00–4:00), and the other 4-
h sleep period was taken at irregular times over a time span
ranging between 7 and 12 d. Their study demonstrated that
if one of the 4-h sleep periods was taken at the same time
each day, the circadian rhythms became stabilized, even when
the other 4-h sleep period was irregularly. However, there
has been little study examining the effect of “anchor sleep,”
after the Miner and Waterhouse study. Further study is
necessary to confirm whether a nighttime nap sustains
circadian rhythms under various nap conditions such as nap
duration and the time when a nap is taken.
Nighttime naps affect subsequent daytime sleep. In short,
daytime sleep, when combined with a nighttime nap, is shorter
in length and has less SWS than daytime sleep alone25). In
a sense this finding seems to indicate a disadvantage.
However, Matsumoto and Harada observed that total hours
of sleep were nearly equal between daytime sleep without a
nap and total sleep (2-h nighttime nap plus subsequent
daytime sleep)12). Sallinen et al. reported a reduction of
SWS in daytime sleep when a 50-min nighttime nap was
taken at around 1:00 and 4:00, but this reduction was not
significant26). The effect of a nighttime nap on subsequent
H TAKEYAMA et al.
Industrial Health 2005, 43, 24–29
daytime sleep might be affected by its length and timing.
However, this finding might be interpreted as a beneficial
effect rather than a negative feature because workers can
then spend time engaged in other activities rather than
Despite these beneficial effects, the existence of the
transient impairment of performance and mood just after
awaking from nap has been established. This effect of
napping is called “sleep inertia”27). The duration of reduced
performance capability reportedly has varied6, 28, 29). The
severity and duration of sleep inertia depends on the length
and quality of sleep, sleep timing, and the length of the prior
time awake15, 30). In the work site, impairment of performance
due to sleep inertia presents a potential problem.
Effective length and timing of the nighttime nap
Previous nap studies have examined naps of various
lengths. Generally, in a case of naps taken during the night
shift, a long nap is not realistic. It is expected that an effective
nap be taken as briefly as possible in the workplace.
Regarding the length of the nap, it is thought that
approximately 90–120 min of a single sleep cycle (falling
asleep to REM sleep) is the most effective. Saito and Sasaki11)
compared 2-h and 1-h naps at 3:00 a.m, and a no-nap
condition. Their results showed that early morning subjective
sleepiness was lower for both nap lengths than no-nap
condition. Roger et al.25) also showed that a 1-h nap at 2:00
a.m. had a beneficial effect on performance. A recent field
study examining much shorter naps (20 min) by Purnell10)
indicated the vigilance tasks are shorter at the end of a night
shift from those brief naps than no napping at all. However,
it is difficult to conclude the adequate nap length necessary
to maintain alertness and performance, because there are
methodological differences among these studies.
In addition to the nap length, the time when a nap is taken
may have an important effect. Matsumoto researched the
effect of 2-h nap taken at 5 different times at night and
suggested that the nap taken at the nadir of circadian rhythm
is most favorable based on his evaluation of changes of
circadian rhythm after the naps22). Gillberg examined the
effect of 1-h naps at either 21:00 or 4:30, with compared to
the no-nap condition9). Their study revealed that both naps
improved performance at the end of night shift (7:00)
compared with no nap, especially the nap taken at 4:30.
Sallinen et al. investigated the effects of naps taken during
the first half of a night shift (around 1:00) for 30 and 50
min, and naps taken during the second half of a night shift
(around 4:00) for 30 and 50 min26). Their results indicated
that 30 or 50 min naps had beneficial effects at both times
while the 50 min naps produced sleep inertia for 10–15 min.
Matsumoto31) reviewed this based on previous studies with
reference to the length and timing of a nighttime nap. He
suggested that a nap of more than 120 min taken in the first
half of a night shift and a nap of approximately 60 min taken
in the second half of a night shift might help maintain alertness
during the post-nap period.
Recently, we also examined the effects of 5 different nap
conditions during night duty regarding nap length and timing
under laboratory conditions32). In short, the effects of naps
taken at 0:00 for 60 and 120 min, naps taken at 4:00 for 60
and 120 min, and no-nap conditions on alertness and
performances were investigated. Both 60 and 120 min naps
in the latter half of the night shift were superior to earlier
naps in terms of sleep quality. However, performance
declined after a 1-h nap taken later during the night shift
due to sleep inertia. Our finding suggested that the
appropriate timing of a short nap must be carefully
considered, such as a 60-min nap during the latter half of a
Workplace nighttime nap studies
Studies conducted on the implementation of nighttime
naps at work sites have been few. Bonnefond et al. carried
out a precise longitudinal study on the workplace13). They
introduced to an organization a new program allowing a 1-
h nap between 23:30 and 3:30, and examined for 1 yr whether
napping during night work would be acceptable to shift
workers, and would not be disruptive to work performance
and the general quality of life of the workers. It was found
that the workers’ vigilance level was higher after the nap,
and the efficacy of napping progressively increased for most
Purnell et al. also carried out a valuable study to identify
the benefits from taking a single nap on the night shift on
subsequent performance and alertness among aircraft
maintenance engineers10). In this study, the worker was given
the opportunity to take a 20-min nap at the workplace between
1:00 and 3:00. It was found that taking a single 20-min nap
during the first night shift significantly improved the speed
of response on a vigilance task at the end of the shift compared
with taking no nap. The 20-min nap in Purnell’s study is
quite short. Therefore, it might be easier than longer naps
when introducing napping into a worksite.
Our study conducted at a fire station indicated that the
workload among the ambulance paramedics was higher than
that among the firefighters called to a large number of
emergency services at night33). Therefore, we carried out
an intervention study, which modified the work system for
THE NIGHTTIME NAP STRATEGIES FOR NIGHT SHIFT WORK
ambulance services during night work so that ambulance
paramedics could be allowed to take a nap, and examined
the effects of this new system on the fatigue and physiological
functions among the ambulance paramedics (data not
published). In short, the traditional system was that the
ambulance paramedics had to deal with all emergency calls
throughout their 24-h shift. In the modified system, two
ambulance paramedics could always take a nap at either
21:00–3:00 or 3:00–8:30 by substituting for another
firefighter. The total length of naps and the longest continuous
nap taken in the new night shift plan were significantly longer
than those in the traditional shift, the number of emergency
dispatches was fewer, and naps was longer in new shift plan
than that in the traditional shift. The beneficial effects of
this new shift plan on performance and subjective data were
also observed. The modified systems in the present study
ensured workers of taking long and continuous naps, and
reduced the adverse effects of night duty. Shoji et al. suggested
that psychological stress due to the likelihood of a forced
and expected awakening could reduce the sleep quality of
emergency physicians34). Ambulance paramedics must be
prepared to dispatch at anytime, when there is an emergency
call. Therefore, knowing they could take uninterrupted naps
affected the quality of their sleep positively.
As reviewed in the studies to date, a nap is an effective
tool to reduce the possible adverse effects from night work.
However, we should recognize the existence of individual
differences regarding the capability of falling asleep.
“Sleepy” and “alert” types have been used as terms indicated
individual differences of sleep propensity35). In short, the
“sleepy” type has the ability to fall asleep more readily. The
“alert” type, however, cannot easily fall asleep and normally
resists sleep. “Morningness” and “eveningness” are well
known as individual differences of chronotype36). “Morning”
type (M-type) has a rigid sleep-wake cycle while the
“evening” type (E-type) has more flexiblity in their time of
sleep. We examined the psycho-physiological effects of the
2-h nighttime nap (2:00–4:00) on M-type and E-type37). In
the M-type, task performance decreased and subjective
feelings of fatigue and anxiety scores decreased after a nap
compared with the no-nap condition. These recovery powers
were not shown in the E-type. Recently, Daurat and Foret
investigated individual differences of sleep-wake pattern
between the nurses who slept during night shift (night-
nappers) and the nurses who did not (nonnight-nappers)38).
The night-nappers had short daytime sleep with polyphasic
patters during daytime after night shift, and nonnight-nappers
had long daytime sleep and took preventive naps to
anticipated sleepiness during night shift. These different
patterns seem to be related by circadian rhythm.
Another important individual aspect of napping is aging.
Härmä suggested that sleep flexibility reduces with aging39).
Sakai and Kogi reported that older workers prefer to go to
bed for a nap before or at midnight, while younger workers
prefer the latter half of the night shift19). Moreover, we need
to be concerned about gender with more and more women
now engaged in night work. These individual differences
are also related to worker lifestyle and are very complex.
Careful design should be the rule when a nap strategy is
introduced at a work site.
Nighttime Napping Strategy in the Future
From the findings of the previous studies, it is difficult to
assert the most effective nighttime nap conditions because
there are methodological differences among the studies and
individual differences. However, there is no doubt that
nighttime napping is an effective tool to improve nightshift
work at a work site.
When a nap strategy is implemented in the work place,
several factors should be considered. First, understanding
the negative effects of nighttime napping as well as the
beneficial effects should be communicated to the workers.
Sleep inertia might be a crucial factor that might cause
occupational accidents in some circumstances. Second,
nighttime napping means that some workers must leave their
job in turns to take a nap when on duty. There are also
limitations in the duration and timing of naps. Therefore, it
is also important to assure a comfortable environment to
guarantee high-efficiency sleep during the allowed rest periods
to managers. As described below, Kogi suggested five rules
of thumb for on-duty napping by nightshift workers40).
• Facilitate the habit of napping at the workplace by
• Select 60–90 min as the napping period where possible
• Select prudent timing in the arrangement of naps so that
the workers can nap in turn during the midnight and early
• Provide a good sleeping environment in a quiet, dark,
and air-conditioned area.
• Collectively plan the nap periods as part of multifaceted
measures for improving shift-work conditions.
Despite the beneficial effects of nighttime napping,
however, there are some obstacles when a napping strategy
is implemented in the workplace. As described above,
H TAKEYAMA et al.
Industrial Health 2005, 43, 24–29
nighttime napping has been introduced in Japanese industries
conventionally. On the other hand, there is the fact that
managers and workers seemed to suffer from the notion that
napping is an indicator of a lack of professional ethics in
other countries15). Providing information to workers and
managers about the risks of lowered alertness reduction and
performance, and the beneficial effectives of nighttime
napping for safety, health, and quality of life might be one
of the solutions to ethical attitudes.
When nighttime napping strategy is introduced in the work
site, we should consider that naps are not the only effective
countermeasure for work performance problems during the
nightshift. Moreover, we should consider how to create an
effective strategy for improving work performance and
promoting safety and health is different for each worksite.
Rosekind et al. pointed out the requirement for attention to
multifactors when a nap strategy is implemented in the work
place41). Recent international regulations by the International
Labour Organization (ILO) have emphasized multifaced
protection and flexibility in working time arrangement42).
Flexibility is important subject in many practical approaches
toward better shift schedules and improving health and
tolerance of shiftworkers43). The ILO showed three essential
steps could be identified as follows: 1) group study of
operational requirements and workers’ need; 2) establishing
feasible options; and 3) consensus building while acting on
conflicting interst44). Participatory methods for the
improvement of working conditions have expanded in Asia45).
The characteristics of these methods are approaches to improve
working conditions focusing on low-cost and using action
checklists and group discussions. These action and
participatory oriented activity heightens motivation among
workers and managers, and helps improve further aspects of
the working conditions combined with these initial
improvements. This activity is important to promote multiple
countermeasures linked with reducing excessive workload
among shift workers. Through these activities, nighttime
napping strategy would be spread more in the workplaces
and play a role as one of the effective tools for improving
working conditions, work performance and safety in the future.
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