SHIFT WORK
Subjective and Objective Measures of Adaptation and Readaptation to Night Work
on an Oil Rig in the North Sea
Bjørn Bjorvatn, MD, PhD1,4; Kristine Stangenes1; Nicolas Øyane, MD1; Knut Forberg1; Arne Lowden, PhD2; Fred Holsten, MD, PhD3,4; Torbjørn Åkerstedt, PhD2
Department of Public Health and Primary Health Care, University of Bergen, Bergen, Norway; 2IPM – National Institute for Psychosocial Medicine,
Karolinska Institute, Stockholm, Sweden; 3Department of Psychiatry, University of Bergen, Bergen, Norway; 4Norwegian Competence Center for Sleep
Disorders, Haukeland University Hospital, Bergen, Norway
1
led to a clear increase in subjective sleepiness and worsening of sleep
parameters. During the week on the day shift, sleepiness and sleep
gradually improved, similar to the improvement seen during night work.
The workers indicated that the day shift was worse than the night shift on
some of the measures, e.g., sleep length was significantly longer during
the night-shift period.
Conclusions: This is one of few studies showing how shift workers in a
real-life setting adjust to night work. Both subjective and objective sleepiness and subjective sleep improved across days. The effects were especially pronounced for the subjective data.
Keywords: Night work, field study, circadian rhythm, subjective ratings,
objective ratings
Study objectives: To study the adaptation and readaptation processes
to 1 week of night work (6:30 PM to 6:30 AM) followed by 1 week of day work
(6:30 AM to 6:30 PM).
Design: Part of a randomized, placebo-controlled, crossover field study.
Here, data from the placebo arm are presented.
Setting: Oil rig in the North Sea. Work schedule: 2 weeks on a 12-hour
shift, with the first week on the night shift and the second week on the
day shift.
Participants: Subjects complaining about problems with adjusting to
shift work. Seventeen workers completed the study.
Interventions: N/A.
Measurements: Subjective and objective measures of sleepiness (Karolinska Sleepiness Scale and simple serial reaction time test) and sleep
(diary and actigraphy).
Results: Both subjective and objective measures improved gradually
during night work. The return to day work after 1 week on the night shift
Citation: Bjorvatn B; Stangenes K; Øyane N; et al. Subjective and objective
measures of adaptation and readaptation to night work on an oil rig in the
north sea. SLEEP 2006;29(6):821-829.
performance, but there are relatively few studies available. In industry, a classic study is that of Bjerner et al, who showed that
errors in meter readings over a period of 20 years in a gas works
had a pronounced peak on the night shift.13 There was also a secondary peak during the afternoon. Similarly, Brown demonstrated that telephone operators connected calls considerably slower
at night.14 Woyczak-Jaroszova found that the speed of spinning
threads in a textile mill went down during the night.15 Most other
studies of performance have used laboratory-type tests and demonstrated, for example, reduced reaction time or poorer mental
arithmetic on the night shift.9,16
Whether there is an adaptation to successive night shifts has
often been discussed. On the whole, this does not seem to be
the case, other than in very small amounts. Thus, sleep duration
and subjective sleepiness do not adjust,17-19 and permanent night
workers do not seem to have markedly longer sleep than do rotating shift workers.17,18 Performance has not been studied much
with respect to adjustment, but accidents seem to increase across
4 night shifts.2 However, simulated shift work (in the laboratory) usually shows an improvement of performance across night
shifts.20 Part of the reason for the lack of adjustment in field studies is very likely the morning exposure to light that prevents a
phase delay.21,22 the lack of interference from light and noise in
the bedroom and social demands in the simulated night work also
very likely may help to improve sleep.
In a previous study, we had the opportunity to carry out a field
study with what might be seen as “laboratory-quality” sleep conditions, that is, without the interference indicated above.23 This
may be an ideal situation for studying the adjustment process to
night work in a real work situation but under conditions optimally
conducive to adjustment. The study was carried out in 7 workers
INTRODUCTION
SHIFT WORK THAT INCLUDES NIGHT WORK IS ASSOCIATED WITH SHORTENED SLEEP AND INCREASED
SLEEPINESS,1 AS WELL AS IMPAIRED PERFORMANCE
and increased accident risk.2 Sleep is reduced by approximately
2 hours in connection with night and morning shifts,3 and mainly
stage 2 and REM are affected.4-6 Sleepiness is particularly increased toward the end of the night shift and is seen in subjective
measures7-9 as well as in electroencephalogram and electrooculogram indicators of sleep intrusions during work.6,10-12
One would expect the increased sleepiness to lead to impaired
Disclosure Statement
This was not an industry supported study. Dr. Redline has served as a scientific advisor for Cypress Bio and Organon. Dr. Bjorvatn has received research support from Lundbeck AS; and has received honorarium for lecture
activities from Lundbeck, Boehringer Ingelheim, and Wyeth. Dr. Holsten has
received research support from Lundbeck AS; has received honorarium for
lecture activities from Lundbeck AS, Lilly, AstraZeneca, Whyet, Aventis Pharma, Bristol Myers, Desitin, GlaxoSmithKline, NoooNordst, Organon, and
Sanofi; and is a member of the advisory board for Lilly, World Schizophrenic,
and Pfizer. Dr. Akerstedt is a consultant for Sanofi-Aventis, Cephalon, and
Organon. Drs. Forberg, Stangenes, Øyane, and Lowden have indicated no
financial conflicts of interest.
Submitted for publication September 8, 2005
Accepted for publication January 29, 2006
Address correspondence to: Bjørn Bjorvatn, MD, PhD, Department of Public
Health and Primary Health Care, University of Bergen, Kalfarveien 31, N5018 Bergen, Norway; Tel: 47 55 58 61 00; Fax: 47 55 58 61 30; E-mail:
[email protected]
SLEEP, Vol. 29, No. 6, 2006
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Adaptation to Night Work at an Oil Rig—Bjorvatn et el
on a North Sea oil rig, working a 14-day 12-hour night shift. The
rig is a self-contained living and working space in which most
work is carried out indoors, with minimal daylight interference,
with good sleeping conditions, and with indoor lighting adjusted
to the work pattern. Thus, the biologic night is turned into day.
The results showed a strong increase in sleepiness the first and
second days but then a rapid adjustment of the sleepiness pattern,
which was rather complete after 4 to 6 days. It also showed a similar rapid adjustment of the sleep pattern. After 14 days offshore,
the workers returned home to daytime life and exhibited large increases in sleepiness and sleep difficulties.
The present study was an extension of the previous one but with
inclusion of objective sleep measurement (actigraphy) and reaction time performance during work. To the best of our knowledge,
this has not been done before. In addition, the shift schedule was
changed to a so-called “rotating shift,” with the first week on the
night shift and the second week on the day shift. This offered the
opportunity to study reentrainment to day work with control for
work activity. In addition, the sample size was greatly increased.
the work hours between the subjects and from day to day, due
to unplanned overtime and individually adjusted work schedules.
This unplanned overtime was not systematically recorded.
Written informed consent was obtained from all subjects. The
study was approved by The Regional National Committee for Research Ethics, and the Norwegian Medicines Agency.
Subjective Measures
Subjective ratings of sleepiness were obtained using the Karolinska Sleepiness Scale25 (KSS and a shortened version of the Accumulated Time with Sleepiness26 (ATS). The KSS is a 9-point
verbally anchored scale with the following steps: 1 = very alert,
3 = alert, 5 = neither alert nor sleepy, 7 = sleepy, but no problem staying awake, and 9 = very sleepy, fighting sleep, effort to
stay awake. The intermediate steps are not anchored verbally. The
subjects rated sleepiness on the KSS every other hour while at
work (at 8:00 PM, 10:00 PM, and so on). The ATS scale is designed
to give an integrated rating representing sleepiness over longer
periods, i.e., accumulated sleepiness (in percentage of time) during the 12-hour shift.26 The subjects were asked: “Did you experience any of the following symptoms: heavy eyelids, feeling
gravel eyed, difficulties in focusing your eyes, irresistible sleepiness, reduced performance, and periods when you were fighting
sleep?” ATS ratings were recorded every day before going to bed
during the 14-day work period. The subjects also gave an overall
rating of the day (1 = very good, 3 = good, 5 = neither good nor
bad, 7 = bad, 9 = very bad). In addition, they recorded their intake
of coffee and tea on a daily basis.
Subjective sleep was obtained with a modified version of the
sleep diary presented by Morin.27 The diary consisted of the subject’s estimates of the prior sleep episode and was recorded daily
for the 14-day work period. The following measures were derived
from the diary: bedtime, light out time, sleep-onset latency, wake
after sleep onset, number of awakenings, early morning awakening (time spent in bed after final wake-up), final wake-up time,
get-up time, total wake time (sleep-onset latency + wake after
sleep onset + early morning awakening), total sleep time, time in
bed, sleep efficiency (total sleep time as a percentage of time in
bed), and an overall rating of the sleep episode (1 = very restless,
5 = very sound).
After the 2-week work period, the subjects were asked in a short
questionnaire how many days they felt it took to adapt to night
work and how many days they felt it took to readapt back to day
work. They were also asked how they rated these 2 weeks of shift
work compared with “regular” shift-work periods (1 = much better than usual, 3 = better than usual, 5 = as usual, 7 = worse than
usual, 9 = much worse than usual).
METHODS
Subjects and Design
All subjects (n = 109) working nights at an oil rig in the North
Sea completed a questionnaire about possible sleep complaints
in relation to shift work (data submitted elsewhere). They were
asked questions about the number of days they needed for adaptation to night work and readaptation back to day work and an
overall rating of their sleep problems in relation to shift work (1 =
no problem, 2 = some, 3 = moderate, 4 = severe, 5 = very severe
problems).24 Based on an evaluation of the questionnaires, 38 subjects were included in the present study. The criteria for inclusion
were problems adjusting to shift work, as indicated by needing
more than 3 days for adaptation/readaptation, or an overall rating
of their sleep problem as more than moderate (> 3).
The subjects were evaluated on 3 consecutive working schedules of
14 days in a randomized crossover design: placebo capsules, melatonin capsules (3 mg), and bright-light treatment. Here we present data
from the placebo condition, in order to focus on the adaptation/readaptation process. Data from the treatment study will be presented
elsewhere. Of the 38 included subjects, 17 completed the study. The
others did not participate or complete for different reasons: did not
want to participate (8 subjects), on sick leave (3), stopped working
this shift schedule (5), and quit or on leave (3), whereas 2 subjects
were dropouts during the study. Mean age of the 17 subjects completing the study was 42 years (range 29-55 years). One was a woman, the
others men. They were in good health, as indicated by their biannual
compulsory medical check-up. They enrolled in the study voluntarily
and were not paid or otherwise compensated for participation. Data
were collected from April 2002 to April 2003. Light levels inside the
oil rig were not recorded in this study. We have previously measured
this on a similar rig, where light levels varied from 20 to 700 lux, with
an estimated average intensity in most areas of 200 to 300 lux.23
The subjects had a work schedule of 2 weeks on a 12-hour shift,
with the first week on the night shift (6:30 PM to 6:30 AM) and
the second week on the day shift (6:30 AM to 6:30 PM). On the
“rotating” day, the workers ended their night shift at 4:00 AM and
started day work at 10:00 AM. After 3 to 4 weeks off work, this
working schedule was repeated. There were small variations in
SLEEP, Vol. 29, No. 6, 2006
Objective Measures
Objective ratings of sleepiness were obtained using a 5-minute
simple serial reaction time test on a PalmTM handheld computer.
This was a modified version of similar tests developed by others.7,28,29 This 5-minute reaction time test has been validated and
compared with longer tests.30 Fifty black squares were displayed
on the screen at squarely distributed intervals (4.75-7.25 seconds)
over 5 minutes. The subject’s task was to respond to the stimuli
by pressing a key to turn off the square. If no response was given
within 1750 milliseconds, a new interval was started. Pressing
the key before the square was displayed, or within 120 millisec822
Adaptation to Night Work at an Oil Rig—Bjorvatn et el
Table 1—Subjective and Objective Sleepiness and Sleep
Sleepiness questionnaire
(n = 17)
KSS mean
Quality of day
Heavy eyelids
Irresistible sleepiness
Reduced performance
Fighting sleep
Coffee or tea intake
Sleep diary (n = 16)
Total sleep time
Total wake time
Sleep efficiency
Sleep-onset latency
Sleep quality
Reaction time (n = 12)
Mean
Median
Actigraphy (n = 14)
Total sleep time
Total wake time
Sleep efficiency
Sleep-onset latency
Conditiona
Daya
F1,16
F6,96
0.02
5.3*
2.3
1.5
1.4
0.4
8.4*
F1,15
8.7*
0.01
0.4
1.0
2.3
F1,11
0.3
0.2
F1,13
23.7***
0.2
0.2
0.1
Table 2—Subjective and Objective Sleepiness using Separate Analyses for the Night and Day Shift
Condition
× Day
F6,96
Daya
Timea
Karolinska Sleepiness Scale score (n = 17)
Subjective sleepiness
F5,80
F5,80
During night work
16.3***
36.4***
During day work
6.8***
5.5*
Reaction time
F2,22
F2,22
22.7***
1.4
6.9***
1.0
1.6
7.9***
7.8***
1.6
5.0**
2.2
9.1***
1.5
0.4
1.7
F5,75
F5,75
3.9**
0.8
6.9***
1.1
7.2***
0.5
1.4
1.1
2.4 p < .10
1.2
F2,22
F2,22
1.6
3.8 p =.055
3.5
3.4
F5,65
F5,65
1.6
1.8
1.7
1.0
1.8
1.0
1.7
0.4
F25,400
3.9***
2.6**
F4,44
Night work (n = 14)
Mean
Median
4.7*
5.3*
2.7
2.8
2.5
1.4
Day work (n = 12)
Mean
Median
2.4
0.7
0.7
0.3
0.6
0.4
Results from 2-way analysis of variance, F values, and p values.
a
Condition refers to night vs day work; day is the day of the week
*
p< .05
**
p< .01
***
p< .001
Statistical Analysis
Data were analyzed using SPSS version 11.5 (SPSS, Inc., Chicago, IL). For a straightforward analysis of overall adaptation to
the shifts, variables with more than 1 measurement per shift were
averaged to yield 1 value per shift. Thus, sleepiness and sleep
were analyzed in a 2-way analysis of variance (ANOVA) with
condition (night work or day work) and day as factors, in order
to compare and follow the adjustment process to night work and
the readaptation back to day work. Lights out and wake-up time
were analyzed separately for the night and day shift using a 1-way
ANOVA with day as factor. Since the work schedule imposed restrictions on sleep opportunity on the rotating day, subjective and
objective sleep data were analyzed during the first 6 days of both
conditions (day 7 excluded). In order to study the within-shift
pattern in detail for KSS and reaction time, another 2-factor repeated-measures ANOVA was carried out, retaining the individual measurements during the shift and employing day and time
of day as factors, separately for the night week and day week.
For the KSS tests, the rotating day was excluded in the ANOVA
analysis due to fewer timepoints. P values were corrected for lack
of compound symmetry using the epsilon correction according to
the Huynh-Feldt procedure. The α level was set at .05. In order to
retain as many subjects as possible in the analysis, missing data
were replaced by careful estimates. If data from, for example,
night 3 were missing, an average of night 2 and 4 was inserted. If
night 7 or day 7 was missing, night or day 6 was inserted instead.
If night or day 1 was missing, night or day2 was inserted. The
total number of missing data that were corrected varied between
1.1% and 3.6%, except for recorded intake of coffee and tea, in
which 8.0% of the data were missing.
Results from 2-way analysis of variance, F values, and p values. KSS
refers to Karolinska Sleepiness Scale.
a
Condition refers to night vs day work; day is the day of the week
*
p< .05
**
p< .01
***
p< .001
onds, caused the response to be discarded and a warning to be
displayed. The software that controlled the internal clock yielded
data with at least a 0.5-millisecond resolution. Another part of the
program calculated the mean and median reaction times, the number of lapses (> 500 milliseconds), and the mean of the 10% best
and slowest reaction times during the 5-minute task. The reaction
time test was performed at 3 timepoints (midnight, 3:00 AM, 6:00
AM) during night 1, 3, and 6 of the night-shift period (week 1) and,
similarly, at 3 timepoints (midday, 3:00 PM, 6:00 PM) during day 1,
3, and 6 of the day-shift period (week 2).
Objective sleep-wake activity was recorded with an Actiwatch
recorder (Cambridge Neurotechnology Ltd, England), which is
a small wrist-worn device, sized 1 × 3 × 3 cm, containing an accelerometer that is optimized for highly effective sleep-wake inference from wrist activity. The Actiwatch has been validated for
documenting longitudinal changes in sleep patterns.31 The sensitivity of the Actiwatch was set to medium. Data were collected in
1-minute epochs and transferred, via an interface, to a computer
and then analyzed (Actigraphy Sleep Analysis 2001, Cambridge
Neurotechnology Ltd). The subjects wore the Actiwatch during
the whole 2-week work period, except when taking a bath or
shower. They were instructed to register the time they went to
bed and the time they got out of bed by pressing a button on the
actigraph. In case the subjects had forgotten to do so, bedtime
or get-up time was obtained from the sleep diary. The following
measures were derived from the actigraph: sleep-onset latency,
wake after sleep onset, early morning awakening, total wake time,
total sleep time, time in bed, and sleep efficiency.
SLEEP, Vol. 29, No. 6, 2006
Day × time
RESULTS
Subjective Measures
Sleepiness
Table 1 and 2 present the results from the ANOVAs. Table 1
shows that there were significant changes in sleepiness across
823
Adaptation to Night Work at an Oil Rig—Bjorvatn et el
6
6
6
5
5
5
Quality of day
4
4
Quality of day
Mean KSS value
Mean KSS value
5
4
6
4
3
3
3
3
2
2
2
1
1
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
11
10
11
12
12
13
2 1
14
13
Heavy eyelids
Per cent of time working
12
12
Heavy eyelids
10
3
6
4
7
5
8
9
6
7
10
11
8
12
9
13
10
14
11
12
13
14
8
6
4
12
13
14
12
13
14
10
Irresistible sleepiness
6
8
Per cent of time working
14
2
5
Irresistible sleepiness
8
Per cent of time working
Per cent of time working
16
4
10
14
18
3
1
14
16
20
2
4
6
2
10
2
8
6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
4
4
1
2
3
4
5
6
7
8
9
6
7
10
11
12
13
14
2
16
10
2
14
12
3
4
5
6
7
8
9
10
11
12
13
14
10
8
6
Reduced performance
4
Per cent of time working
Per cent of time working
16
2
8
6
1
2
3
4
5
8
9
10
11
10
Fighting sleep
4
8
Per cent of time working
18
Per cent of time working
20
1
12
Fighting sleep
Reduced performance
14
2
2
10
1
2
3
4
5
6
8
7
8
9
10
11
12
13
14
Days
6
4
6
1
2
3
4
5
6
4
7
8
9
6
7
10
11
12
13
14
Days
2
2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Days
1
2
3
4
5
8
9
10
11
Days
Figure 1—Mean ratings (± SEM) from the subjective sleepiness questionnaire. The first 7 days represent values during the nightwork period, whereas the last 7 days represent values during the following day-work period. A dotted line separates the 2 work
periods. N = 17. Higher Karolinska Sleepiness Scale (KSS) values indicate higher sleepiness. Higher values on the quality-of-day
scale indicate worse quality.
days on all measured parameters. During the first week of night
work, sleepiness was gradually reduced. Then, when the workers
shifted back to day work, sleepiness was again increased to high
levels. During the second week (day work), and similar to the
night-work week, sleepiness was gradually reduced toward the
end of the working period (Figure 1).
SLEEP, Vol. 29, No. 6, 2006
In addition, KSS ratings at different timepoints were analyzed,
separately for the night shift and day shift (Table 2, Figure 2).
During night work, sleepiness was significantly reduced from day
to day, whereas the time-of-day analysis showed that sleepiness
steadily increased from 8 PM to 6 AM (Table 2, Figure 2). In addition, there was a significant interaction between day and time,
824
Adaptation to Night Work at an Oil Rig—Bjorvatn et el
Figure 2
Sleep Diary
Night shift (first week)
8
4
Most sleep parameters changed significantly across days (Table 1, Figure 3). For total sleep time, there was also a significant effect of condition, that is, the subjects slept longer during
the night-work period (386 minutes, SEM = 13) than during the
following day-work period (345 minutes, SEM = 9)(Figure 3).
During night work, sleep length was shortened following the first
night (less than 6 hours of sleep), but, thereafter, total sleep time
was approximately 6.5 hours per night. The rotating day was excluded from the statistical analysis, since sleep here was truncated
due to the work schedule. As Figure 3 shows, less than 4 hours
of sleep was seen during the rotating day. Interestingly, sleep was
shortened to less than 6 hours for many days following the shift
back to day work (Figure 3). For total wake time and sleep efficiency, values improved significantly across days, both during the
night-work and day-work periods (Table 1, Figure 3). There were
no significant differences between the conditions (night or day
work). Sleep-onset latency did not differ across days or between
conditions (Table 1).
There was no significant change across days in when the workers turned off the lights to go to sleep during the night shift (F5,75
= 1.1) or during the day shift (F5,75 = 2.2, p = .07). Wake-up time
during the night shift did not change either (F5,75 = = 1.5), but,
during the day shift, a significant delay was seen (F5,75 = 3.8, p <
.01). Figure 4 shows lights-out and wake-up times during both the
night and day shifts.
3
Short Questionnaire
7
Mean KSS value
6
5
4
3
2
night 1
night 2
night 3
night 4
night 5
night 6
night 7
Individual time points
Day shift (second week)
8
7
Mean KSS value
6
5
The subjects reported significantly fewer days for adaptation
to night work than for adaptation back to day work (2.7 versus
4.4, p = .002, t-test). The subjects rated these 2 weeks of shift
work compared with “regular” shift-work periods. The score was
5.6 (SD 1.8), indicating that the shift-work period was rated “as
usual.”
2
day 1
day 2
Figure 2
day 3
day 4
day 5
day 6
day 7
Individual time points
Figure 2—Mean ratings (± SEM) of sleepiness according to
Karolinska Sleepiness Scale (KSS) at individual time points
(every other hour while at work) during the night work week
(top panel) and day work week (bottom panel). The different
days are separated by dotted lines. N = 17.
Objective Measures
Sleepiness—Reaction Time Test
Due to missing data and technical problems with the Palm
computer, fewer subjects (n = 14 night shift, n = 12 day shift)
completed the reaction time tests. Similar to the subjective sleepiness data, no consistent differences in reaction times were seen
between the night-shift and day-shift conditions (Table 1, Figure 5). The second ANOVA that included separate analysis for
the night and day shift showed a significant reduction in reaction
time across days for the night shift but not for the day shift (Table
2, Figure 5). During the day shift, no significant changes were
seen across days, time of day, or interaction between day and time
(Table 2, Figure 5).
indicating that the rate of increase in sleepiness across the night
fell during the work period (Figure 2).
During day work, significant changes across days and between
individual timepoints were seen, as was a significant interaction
between day and time (Table 2, Figure 2). Similar to findings from
the night work, sleepiness decreased from day to day (Figure 2).
The first day after shifting back to day work, sleepiness was highest early in the day (at 10 AM), whereas, during the following days,
sleepiness showed an increasing trend with highest sleepiness in
the late afternoon (Figure 2).
On most sleepiness measures (Table 1), no significant difference
was seen between the night- and day-shift conditions. In contrast,
the workers reported that the quality of day was significantly reduced during the day-work period compared with during night
work (Table 1, Figure 1).
There was a significantly higher intake of coffee or tea during
the day shift compared with the night shift (Table 1). The intake
ranged from 3.2 to 3.8 cups per day during night work, whereas,
during day work, the intake ranged from 3.8 to 4.2 cups per day.
No difference across days was seen.
SLEEP, Vol. 29, No. 6, 2006
Sleep—Actigraphy
Objective sleep showed similar values as subjective sleep, but
there were no significant differences across days on the measured
parameters (Table 1, Figure 3). As for subjective sleep data, total sleep time was clearly longer during the night shift compared
with during the day shift (Table 1, Figure 3). Total wake time,
sleep efficiency, or sleep latency did not show any significant
changes across condition (Table 1, Figure 3).
825
Adaptation to Night Work at an Oil Rig—Bjorvatn et el
Sleep diary
450
420
360
360
330
330
300
300
270
270
330
300
270
240
210
210
2
1
3
2
4
5
3
4
6
5
7
8
6
7
9
8
10
9
11
12
13
10
11
390
360
330
300
270
240
1
14
12
13
210
2
3
4
1
14
5
2
6
3
7
4
8
5
9
6
10
11
12
7
8
9
10
11
12
7
8
9
10
11
12
7
8
9
13
10
14
11
12
13
14
12
13
14
12
13
14
120
120
120
120
105
Total wake time (minutes)
105
90
90
75
60
75
45
60
30
45
90
105
75
90
60
45
30
15
15
30
1
100
2
1
3
2
4
3
5
4
6
5
7
8
6
7
9
8
10
9
11
10
12
13
11
12
75
60
45
30
1 15 2
14
13
95
100
3
4
1
14
5
2
6
3
7
4
8
5
9
6
13
10
14
11
95
100
90
100
90
80
75
85
85
95
80
90
75
70
70
65
65
80
75
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sleep efficiency (%)
Sleep efficiency (%)
90
95
85
70
Total wake time (minutes)
105
Totaltime
wake
time (minutes)
Total wake
(minutes)
360
240
240
Sleep
efficiency (%)
Sleep efficiency
(%)
390
Total sleep time (minutes)
Total sleep time (minutes)
Totalsleep
sleep time
time (minutes)
Total
(minutes)
390
390
15
420
420
1
Actigraphy
450
420
210
Actigraphy
450
Sleep diary
450
85
80
75
1
70
2
3
4
5
6
Days
7
8
9
13
14
Days
65
65
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1
Days
2
3
4
5
6
10
11
Days
Figure 3——Subjective (diary) and objective (actigraphy) sleep data (± SEM). The first 7 days represent data during the night-work
period, whereas the last 7 days represent data during the following day-work period. A dotted line separates the 2 work periods. N
= 16.
DISCUSSION
The adjustment is similar to what was seen in our previous
study,23 but contrasts with what is usually the case in more conventional shift-work situations in which sleepiness and sleep do not
seem to improve much.19,32-35 The adjustment seems more similar
to what is seen in laboratory simulations of night work.20,36 Part
of the reason for the lack of adjustment in traditional field studies
is very likely the morning exposure to light that prevents a phase
delay.21,22,37 In the laboratory studies, the lack of interference from
Both subjective and objective measures of sleepiness and subjective sleep gradually improved during the night-work week. The
return to day work after 1 week on the night shift led to a clear
increase in subjective sleepiness and worsening of sleep parameters. During the week on day shift, sleepiness and sleep gradually
improved, similar to the improvement seen during night work.
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Adaptation to Night Work at an Oil Rig—Bjorvatn et el
400
00:00
375
22:00
20:00
350
18:00
16:00
325
lights out
wake-up
14:00
300
Time of Day
10:00
08:00
06:00
04:00
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Mean Reaction Time
12:00
Days
night 1
night 3
night 6
day 1
day 3
day 6
Individual time points
Figure 5—Mean reaction times (± SEM) on PalmTM handheld
computers at individual timepoints during the night shift (at
Fig. 5
midnight,
3:00 AM and 6:00 AM during night 1, 3, and 6) and day
shift (at midday, 3:00 PM and 6:00 PM during day 1, 3, and 6).
The different days are separated by dotted lines. N = 12.
Figure 4—Mean lights out and wake-up time (± SEM). The first
7 days represent values during the night-work period, whereas
the last 7 days represent values during the following day-work
period. N = 16 (except some missing data on night 7 and day 7).
fact that most people have a biologic clock running at a period
slightly longer than 24 hours.40 On the other hand, with 12-hour
shifts, a shift from day to night work would seem to be as much
of a delay as a shift to day work. But, inspection of the pattern of
adjustment of KSS indicates that the pattern on the last night still
was one of a rise during the shift (from 8 PM to 6 AM). A full reversal was clearly not seen, since normal day work usually shows
a U shape with low values in the middle of the waking span.7
Thus, one might suspect that full adjustment had not occurred, at
least not in terms of pattern. On the day shift, however, the fall
during the first day gradually became more of a U shape toward
the last day, suggesting a return to a normal daytime pattern. In
the present case, external light exposure was virtually nonexistent during daytime, and, thus, readaptation was not facilitated by
morning light exposure. Direct comparisons between the night
and day shifts are complicated, since the day shift follows a week
of 7 consecutive night shifts, likely to induce cumulative sleep
deprivation. Also, the workers suffer from acute sleep deprivation during the rotating day preceding the day shift. These factors
influence how the workers adjust to the day shift.
However, we would like to point out that, even though the dayshift period may be worse, both sleepiness and sleep were compromised during the first days and nights on the night shift. That
is, the present study clearly shows that night work is problematic
for these offshore workers. It is important to note that the present
study only examined shift workers complaining about night work.
Many shift workers claim that they do not suffer from sleepiness
or disturbed sleep. Few studies have examined these workers in
detail, in order to look for possible discordance between subjective and objective measures. One recent study showed that
satisfaction with the shift schedule reflected how well the shift
workers coped with the schedule, suggesting that the increase in
sleep-wake problems for the dissatisfied workers may be related
to increased sensitivity to curtailed and displaced sleep.41
The subjective and objective measures of sleepiness and sleep
did not show complete agreement. The results were clearer and
more significant for the subjective data. There are many possible
reasons for this. Actigraphy is considered a valid form of objective sleep recording, and, even though it may overestimate sleep,
it has been shown to correspond well with sleep diaries and poly-
light and noise in the bedroom and the absence of social demands
also may help to improve sleep. Working offshore at an oil rig
may provide similar and more optimal conditions for adjustment,
since workers are not leaving the rigs between shifts. Our previous study23 also showed a gradual adjustment, but across 14 days.
This study, however, was small (7 subjects) and only investigated
subjective measures. Thus, the present study extends the previous
findings that improvement in sleep and sleepiness is possible in
real-life night-work settings. Other studies have looked at adaptation to night work using circadian markers. Barnes et al showed
that oil rig workers on a 2-week night shift adapted to night work
within a week, as measured by the circadian phase marker melatonin.38 In a similar shift schedule, as in the present study, Gibbs
et al showed that oil rig workers adapted to the night shift during the first week, whereas there were large individual variations
seen during the day shift, using a urinary melatonin metabolite as
circadian marker.39 They concluded that subjects did adapt to the
night shift but not back to day shift.
One convincing piece of evidence of an adjustment to night
work in the present study was the clear increase in sleepiness and
worsening of sleep following return to day work. If adjustment to
night work had not taken place, we would have expected a fairly
easy return to day work. For most measures, there were no significant differences between the night- and day-work conditions, indicating that both shifts were problematic. Interestingly, for some
measures such as quality of day and total sleep time, the day shift
was considered worse. Sleep length was clearly shortened for several days following the shift back to day work. The workers also
reported that they felt it took fewer days to adapt to night work
than to readapt back to day work: 2.7 versus 4.4 days. This was
also reflected in the disappearance of symptoms like “heavy eyelids” after 3 to 4 night shifts, whereas, during the day shift, such
symptoms were maintained at higher levels throughout the week.
This is consistent with the complaints of many offshore workers,
who claim that the problem is mainly during the readaptation period (personal communication).
One reason why it may be easier to adapt to night work than to
readapt back to day work is that it is easier to phase delay than
to phase advance the circadian rhythm. This is in part due to the
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Adaptation to Night Work at an Oil Rig—Bjorvatn et el
somnography.31,42 The performance measure chosen was a simple
serial reaction time task, which has been identified as being sensitive to sleepiness-inducing situations.43-45 Even though the workers
reported increased sleepiness in the present study, they performed
relatively well on the performance test. In fact, during day work,
no significant changes were seen across days or time of day. One
may argue that a 5-minute reaction time test is too short, as most
other researchers have used tests lasting 10 minutes.43-45 Recently,
a direct comparison between a 10-minute and a 5-minute reaction
time test showed comparable results following 28 hours of sustained wakefulness.30 A shorter test was used in the present study
for logistic purposes but may have masked possible sleepinessinduced performance decrements. The reaction test was also used
during the second half of the shift and only a few nights or days,
making direct comparisons with subjective sleepiness difficult.
In conclusion, this is one of few studies showing that the sleepiness and sleep of shift workers in a real-life setting improve during
consecutive night shifts. Both subjective and objective sleepiness
and subjective sleep measures showed improvement. The effects
were especially pronounced for the subjective data.
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ACKNOWLEDGEMENTS
We are very grateful to nurses Heidi Eliassen, Alf Iversen, Atle
Nordfonn og Erik Lea at the oil rig “Snorre” for all the help and
enthusiasm in planning and executing these experiments.
The study was supported by a grant from the Norwegian oil
company Hydro AS.
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