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OPEN

Association of sleep quality
with duty hours, mental health,and medical errors among Japanese
postgraduate residents:a cross‑sectional study
Kazuya Nagasaki 1, Hiroyuki Kobayashi 1*, Yuji Nishizaki 2, Masaru Kurihara 3,
Takashi Watari 4, Taro Shimizu 5, Yu Yamamoto 6, Kiyoshi Shikino 7, Sho Fukui 8,
Sho Nishiguchi 9, Kohta Katayama 10,11 & Yasuharu Tokuda 12,13
Long duty hours (DH) impair sleep and negatively affect residents’ health and medical safety. Thiscross-sectional study investigated the association among residents’ DH, sleep duration, insomnia,
sleep impairment, depressive symptoms, and self-reported medical errors among 5579 residents inJapan who completed the General Medicine In-Training Examination (2021) and participated in the
training-environment survey. Weekly DH was classified under seven categories. Sleep duration andinsomnia symptoms, from the Athens Insomnia Scale, were analysed to determine sleep impairment;
depressive symptoms and medical errors were self-reported. Among 5095 residents, 15.5% slept < 5 h/day, and 26.7% had insomnia. In multivariable analysis, compared with ≥ 60 and < 70, DH ≥ 90 h/week
associated with shorter sleep duration and worsen insomnia symptoms. Shorter durations of sleep andmore intense symptoms of insomnia were associated with increased depressive symptoms. Medical
errors increased only among residents with insomnia, but were not associated with sleep duration.
DH > 90 h/week could lead to shorter sleep duration, worsen insomnia symptoms, and negatively
impact well-being and medical safety. There was no significant association between sleep durationand medical errors; however, insomnia conferred an increased risk of medical errors. Limiting DH for
residents to avoid excessive workload can help improve resident sleep, enhance resident well-being,
and potentially reduce insomnia-associated medical errors.
Long working hours can adversely affect health and increase the risk of mental disorders, suicide, sleep impairment, and cardiovascular ­disease1–3. Japanese resident-physicians often work long hours, and resident doctors intheir 20s work, on average, 76.1 h/week4. A new legislation to limit overtime work to 1860 h per year (i.e., weekly
duty hour [DH] limit: ~ 80 h/week) will apply from 2024 in Japan and was enacted based on the health–education
balance and work regulations in other ­countries5. Nonetheless, the DH limit is significantly longer than thosefor general physicians (60 h/week) and general workers (48 h/week) and thus instil concern about the potential
negative effects of work hours on health and lifestyle. A cross-sectional study on mental health among approximately one-third of the Japanese residents found that working > 90 h/week increased depression, burnout, and1

Department of Internal Medicine, Mito Kyodo General Hospital, University of Tsukuba, 3‑2‑7, Miyamachi,
Mito, Ibaraki 310‑0015, Japan. 2Division of Medical Education, Juntendo University School of Medicine, Tokyo,
Japan. 3Department of Patient Safety, Nagoya University Hospital, Aichi, Japan. 4General Medicine Center,
Shimane University Hospital, Shimane, Japan. 5Department of Diagnostic and Generalist Medicine, Dokkyo
Medical University Hospital, Tochigi, Japan. 6Division of General Medicine, Center for Community Medicine,
Jichi Medical University, Tochigi, Japan. 7Department of General Medicine, Chiba University Hospital, Chiba,
Japan. 8Department of Emergency and General Medicine, Kyorin University, Tokyo, Japan. 9Department of General
Internal Medicine, Shonan Kamakura General Hospital, Kamakura, Japan. 10Department of General Internal
Medicine, St. Marianna University School of Medicine, Kanagawa, Japan. 11Department of Clinical Epidemiology,
Graduate School of Medicine, Fukushima Medical University, Fukushima, Japan. 12Muribushi Okinawa for Teaching
Hospitals, Okinawa, Japan. 13Tokyo Foundation for Policy Research, Tokyo, Japan. *email: hrkoba1@gmail.com
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www.nature.com/scientificreports/­stress6. Longer DHs are associated with greater increases in depressive s­ ymptoms7,8. Decreasing DH or setting
appropriate thresholds may improve mental health. Despite the reported benefits of DH restrictions on sleeprelated impairment, the extent to which limiting weekly DH decreases resident sleep deprivation and improves
well-being and patient safety remains unclear.
Few studies have investigated the association of weekly DH with sleep duration; however, most have consistently demonstrated that longer DH correlated with shorter sleep duration. A study involving the general workforce made a comparison between those working > 55 h/week and those working 35–40 h/week, revealing thatthe former group had a 1.98 times higher likelihood of sleeping < 7 ­h9. In 2011, a study of 1241 Japanese residents
reported that sleep duration decreased as weekly DHs i­ncreased7. The study further indicated that almost all
residents got < 7 h of sleep, and those working > 80 h/week often slept < 6 h, with an average of 5.67 h. Anotherstudy found a strong negative correlation between resident sleep duration and weekly D­ H10. In randomized
controlled trials, shortened single-shift work or protected sleep time resulted in improved sleep duration anddecreased ­sleepiness11–13. To our knowledge, no study has investigated the association between insomnia-related
symptoms and weekly DHs in residents.
Inadequate sleep affects attention and working ­memory14,15, thereby eliciting concern about patient safety. A
large body of evidence indicates that both acute and chronic sleep deprivation affect cognitive ­performance14,16.
Among healthy adults, chronic sleep deprivation, with sleep duration < 6 h/day, caused cognitive impairment
equivalent to that induced by 2 days of ­sleeplessness14. In a large-sample study of physicians, severe sleep-related
impairment was associated with increased medical e­ rrors17. However, decreasing only the shift-work duration
produced conflicting results of increasing/decreasing medical e­ rrors18,19.
Despite the posited association between sleep impairment and poor mental health, only fewer large-scalestudies have investigated this relationship. Resident burnout, which overlaps with depression, can impair academic performance and reduce the quality of medical c­ are20,21. Trockel et al.17 found a strong correlation between
sleep-related impairment and burnout in a large sample of physicians. Other studies revealed a link between sleep
deprivation and ­burnout22,23. However, a large prospective observational study found no effect of differences in
DH or daily sleep duration on resident b­ urnout24.
Japan’s current focus is on weekly DH limits to promote the health and sleep of residents. However, previous
studies on the impact of DH on resident sleep have mainly examined interventions that improve per-shift work
rather than weekly ­DH25 and were mostly conducted in the United States after the introduction of the weekly“80-h rule” in ­200426; therefore, those studies did not provide sufficient evidence of the optimal duration of weekly
DH for sufficient sleep in Japan. This large-sample study examined the association between resident weekly DH
and sleep-related variables, sleep duration, and insomnia symptoms among residents in Japan and assessed theimpact of sleep-related impairments on residents’ depressive symptoms and self-reported medical errors.

Methods

Study settingThis national, cross-sectional study enrolled postgraduate residents who completed the General Medicine Intraining Examination (GM-ITE) in Japan between January 17 and January 30, 2022 and subsequently participatedin a training environment survey.
The research consent form, shared with the survey, ascertained voluntary participation and averredanonymized responses. All participants read and signed the informed consent document before the survey. The
study was approved by the Ethical Review Committee of JAMEP (No. 22-1-2) and was conducted in accordancewith the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guidelines.
Additionally, this study was conducted in accordance with the Declaration of Helsinki.

Participants

The participants were postgraduate year (PGY)-1 and PGY-2 residents who completed the GM-ITE and voluntarily participated. Respondents without complete data on DH and sleep-related variables were excluded fromthe analysis.
After medical school, Japanese residents undergo 2-year residency training to acquire general medical skills at
community and university hospital training sites based on a preferential hospital-matching system for mandatory(internal medicine, emergency medicine, surgery, paediatrics, obstetrics and gynaecology, psychiatry, and community medicine) and elective r­ otations23,27. During their residency training, residents are required to perform
multiple emergency duty (ED) shifts each month, in addition to their regular departmental rotation training.

Measurements

Resident characteristics
Participant characteristics, including age, sex, PGY, hospital type (community or university hospital), average
number of assigned inpatients (i.e., the number of inpatients a resident is responsible for at one time), numberof ED duties per month, and self-study time (SST) per day, were obtained in the survey. The translated survey
questions are presented in the supplementary material (Appendix 1). Resident participants completed an electronic survey immediately following the GM-ITE.
Sleep duration and insomnia symptoms
The average daily sleep duration in the previous month was classified as: ≤ 4, 5, 6, 7, 8, and ≥ 9 h. Insomnia-related
symptoms were assessed with the Athens Insomnia Scale (AIS; Japanese version)28, which comprises eight questions on insomnia symptoms experienced in the previous month (each scored 0–3 points and total score of 24

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points). A score ≥ 6 was a positive indicator of insomnia, and the severity of insomnia symptoms was graded by
the AIS score as absence (0–5 points), mild (6–9 points), moderate (10–15 points), and severe (16–24 points)29.
Weekly DH
Residents were asked about their weekly DH during the entire training period including hours worked on weekdays, weekends, and night emergency department (ED) duties. They reported their DHs using a 10-category scale,
which was then categorized into seven groups: 1 (C1, < 50 h), 2 (C2, ≥ 50 and < 60 h), 3 (C3, ≥ 60 and < 70 h), 4(C4, ≥ 70 and < 80 h), 5 (C5, ≥ 80 and < 90 h), 6 (C6, ≥ 90 and < 100 h), and 7 (C7, ≥ 100 h).
Depressive symptoms
Depressive symptoms were assessed with the 2-item Patient Health Questionnaire (PHQ-2; Japanese version)30,
which ascertains loss of interest or pleasure and depressive mood over the past 2 weeks. Positive depression
screening was indicated by a “yes” response to either of the two questions. The PHQ has adequate diagnostic
capability to identify clinical depression (sensitivity 76%; specificity 87%)31.
Medical errors
Residents were asked to report the number of their medical errors (which occurred 0, 1, 2, 3, 4, or ≥ 5 times)
during the past year that caused minor or severe disabilities or disadvantages for patients.

Statistical analysis

The characteristics of residents and their work environments were summarized by the weekly DH category,with category-specific data on sleep duration and insomnia symptoms. Associations between resident sleep andDH, mental health, and medical errors were analysed using multivariable analysis wherein Category 3 (≥ 60
and < 70 h) was the reference group, because 60 h/week is the DH limit for general p­ hysicians5. These analyses
are repeated, categorized by PGY.
Weekly DH and sleep‑related variables
The association between weekly DH and sleep duration and between weekly DH and insomnia symptoms wereanalysed in a proportional odds regression model. To account for variability, the OR and 95% CI were estimated using generalized estimating equations (GEE) with the working independent correlation within hospitals.
The analysis was adjusted for age, sex, PGY, hospital type, ED duties, assigned inpatients, SST, and depressive
symptoms.
Resident sleep and depressive symptoms
The association between sleep-related variables (sleep duration and insomnia symptoms) and depressive symptoms was analysed using modified Poisson regression models with GEEs to account for hospital variability after
adjustment for age, sex, PGY, hospital type, weekly DH, ED duties, assigned patients, and SST.
Resident sleep and medical errors
The association between sleep-related variables (sleep duration and insomnia symptoms) and medical errors(minor or severe) was analysed using proportional odds regression models with GEE to account for hospitalvariability after adjusting for age, sex, PGY, hospital type, weekly DH, ED duty, assigned patients, SST, anddepressive symptoms.
In addition, structural equation models were constructed to test the direct and indirect associations between
weekly DH, sleep-related variables, depressive symptoms, and severe medical errors, employing standardized
estimates of effect and standard goodness-of-fit measures. These analyses were conducted using STATA version
15 (STATA Corporation, College Station, TX, USA).

Results

Basic characteristics

A total of 7681 residents completed the GM-ITE, and 5579 agreed to participate in this study. After excluding
respondents with incomplete data, the final sample comprised 5095 residents.
The basic characteristics of the participants are detailed in Table 1. Of the participants, 31.8% were women,
49.5% were in PGY-2, and 18.3% were from university hospitals. The average age was 26 years (29.3%), and 11.3%
were ≥ 30 years. Residents were most frequently assigned to night ED duties 3–5 times per month (71.4%), had
5–9 inpatients (55.1%), and spent 0–30 min/day (41.0%) in self-study.

Sleep duration and insomnia symptoms

Figure 1 presents DH-category-specific data on resident sleep duration and insomnia symptoms. Approximately
half of the residents (50.2%) reported sleeping 6 h/day, whereas 15.5% slept < 5 h/day. The group of residents
with DH ≥ 80 h/week (C6, C7, and C8) comprised a higher proportion (20.6%) of participants who slept < 5 h/day, as compared to the proportion (12.7%) in the group of residents who worked < 60 h/week (C1 and C2).
The mean AIS score was 3.96 ± 3.7; 26.7% of residents had insomnia symptoms. Among them 20.2%, 5.6%,
and 0.9% had mild, moderate, and severe symptoms, respectively. A significant proportion of residents who
worked ≥ 80 h/week (30.2%) experienced moderate (6.2%) or severe (1.5%) insomnia symptoms.

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Weekly duty hours*
Characteristics

Total

C1

C2

C3

C4

C5

C6

C7

N = 5095

N = 754

N = 1246

N = 1177

N = 689

N = 770

N = 249

N = 210Age (%)
24

3.3

2.4

2.7

4.0

5.1

2.3

2.8

4.3

25

20.4

17.5

20.0

23.9

21.4

19.4

19.7

14.8

26

29.4

25.2

30.6

29.1

29.5

32.1

29.3

28.1

27

20.9

21.6

20.4

20.5

21.4

20.4

23.7

20.5

28

9.5

11.0

8.8

8.9

8.7

10.9

9.2

10.0

29

5.1

6.1

5.5

4.2

4.9

4.3

4.4

8.6
13.8

30 or more

11.4

16.2

12.0

9.4

9.0

10.6

10.8

PGY-2 (%)†

49.5

51.1

49.2

46.0

50.9

48.7

55.0

57.6

Male sex (%)

68.2

64.6

69.8

70.4

66.5

64.4

67.9

78.1

Community hospital (%)‡

81.7

75.2

82.0

84.0

84.5

80.1

84.3

84.3

ED duty per month (%)
None

3.5

9.6

4.3

1.8

2.0

1.9

0.4

1.4

1–2

15.5

28.2

17.7

13.0

10.3

12.7

8.1

6.7

3–5

71.6

58.1

72.9

76.8

76.9

72.7

71.8

61.0

6 or more

8.8

2.8

4.4

8.1

10.5

12.5

19.0

29.5

Unknown

0.6

1.3

0.6

0.3

0.3

0.1

0.8

1.4

Assigned inpatient (%)
0–4

30.3

46.2

35.7

25.4

24.0

25.3

21.7

17.6

5–9

55.2

46.1

54.5

59.7

58.2

57.8

53.0

50.5

10–14

8.9

4.0

6.4

9.1

11.2

11.5

14.1

17.1

15 or more

2.7

0.9

0.9

2.6

3.2

3.4

7.6

11.0

Unknown

2.9

2.8

2.5

3.2

3.3

2.0

3.6

3.8

Self-study time per day (%)
None

3.5

3.8

3.2

3.8

2.8

3.4

2.0

8.1

0–30 min

41.1

50.5

43.5

38.6

37.9

37.1

39.5

35.2

31–60 min

39.3

34.6

41.0

39.4

40.6

42.3

38.3

31.9

61–90 min

12.4

9.4

9.7

15.2

14.3

12.6

15.3

14.3

91 min or more

3.5

1.6

2.6

3.1

4.4

4.7

4.8

10.5

Table 1.  Characteristics of the participants categorized by weekly duty hours. *The categories of weekly
duty hours were as follows: Category 1 (C1, < 50 h), Category 2 (C2, ≥ 50 and < 60 h), Category 3 (C3, ≥ 60
and < 70 h), Category 4 (C4, ≥ 70 and < 80 h), Category 5 (C5, ≥ 80 and < 90 h), Category 6 (C6, ≥ 90
and < 100 h), and Category 7 (C7, ≥ 100 h). † Postgraduate residents are either Postgraduate year (PGY)-1 or
PGY-2. ‡ Training hospitals are divided into community and university hospitals.

Weekly DH and sleep‑related variables

Table 2 presents the OR of DH in the univariable and multivariable proportional odds models for sleep durationand insomnia symptoms. Multivariable analysis revealed that residents who worked < 60 h/week (Categories 1
and 2) had longer sleep duration than residents who worked ≥ 60 and < 70 h/week (Category 3; reference group).
Conversely, residents who worked ≥ 80 h/week (Categories 5, 6, and 7) had a shorter sleep duration than residents in the reference group. The severity of insomnia symptoms was lower in the group that worked < 50 h/
week (Category 1) and higher in the group that worked ≥ 90 and < 100 h/week (Category 6), as compared to thereference group.
The detailed results of the multivariable proportional odds models are presented in Supplementary Table 1.
The analysis revealed that older, male, and PGY-1 residents were more likely to experience shorter sleep durations
and insomnia symptoms. University hospital residents showed a higher likelihood of experiencing insomnia
symptoms than community hospital residents. The number of emergency duty shifts did not significantly affectresident sleep; however, having more inpatients under their care reduced sleep duration. Increased self-study
was associated with shorter sleep duration but fewer insomnia symptoms. Finally, depressive symptoms werelinked to shorter sleep duration and a significantly higher likelihood of insomnia symptoms.

Resident sleep and depressive symptoms

Table 3 presents the prevalence ratio (PR) of sleep-related variables in the univariable and multivariable modified
Poisson regression models for the presence of depressive symptoms. The prevalence of depressive symptoms by
category of sleep-related variables is shown in Supplementary Table 2. The prevalence of depressive symptoms
was 24.4%. The results of the multivariable analysis indicate that individuals who slept < 4 or 5 h/day had ahigher prevalence of depressive symptoms than those who slept 7 h/day (PR [95% CI]: 2.12 [1.66–2.72] and 1.56
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Sleep duration

Per c en t o f Par t i c i p an t s

100%

80%
>9 hours
8 hours60%

7 hours
6 hours40%

5 hours<4 hours

20%

0%

C1

C2

C3
C4
C5
Weekly duty hour

C6

C7

Insomnia symptoms

Per c en t o f Par t i c i p an t s

100%

80%

Severe60%

Moderate
Mild40%

Absence

20%

0%

C1

C2

C3
C4
C5
Weekly duty hour

C6

C7

Figure 1.  Sleep-related variables categorized by weekly duty hours. The categories of weekly duty hours wereas follows: Category 1 (C1, < 50 h), Category 2 (C2, ≥ 50 and < 60 h), Category 3 (C3, ≥ 60 and < 70 h), Category
4 (C4, ≥ 70 and < 80 h), Category 5 (C5, ≥ 80 and < 90 h), Category 6 (C6, ≥ 90 and < 100 h), and Category 7(C7, ≥ 100 h). *On the Athens Insomnia Scale, insomnia severity is classified as follows: 0–5 points, “absence”;
6–9 points, “mild”; 10–15 points, “moderate”; and 16–24 points, “severe”.

[1.37–1.78], respectively). Compared to those without insomnia symptoms, residents who experienced insomnia
symptoms were 3–5 times more likely to have depressive symptoms.

Resident sleep and medical errors

Table 4 presents the OR of sleep-related variables in the univariable and multivariable proportional odds models
for both minor and severe medical errors. The number of medical errors per year per category of sleep–related
variables is shown in Supplementary Table 3. Overall, 45.9% and 18.7% of residents had experienced at least
one minor and one severe error, respectively, in the past year. Multivariate analysis revealed no clear association between medical errors and sleep duration, although the presence of insomnia symptoms was linked to anincrease in medical errors. Compared to residents without insomnia symptoms, those with mild and moderate
to severe insomnia symptoms had higher rates of minor medical errors (OR [95% CI]: 1.21 [1.05–1.40] and1.42 [1.13–1.81], respectively), whereas those with moderate to severe insomnia symptoms had higher rates ofsevere errors (1.92 [1.45–2.56]).

Analyses categorized by postgraduate year (PGY)

The results of multivariable analyses assessing the associations between resident sleep and DH, mental health, andmedical errors, categorized by PGY, were presented in Supplementary Tables 3, 4, and5. Although no significant
differences were observed in the association between DH and sleep duration across different PGYs, insomnia
symptoms were positively associated with DH exclusively in PGY-1 residents, unlike in PGY-2 residents. The
associations between sleep-related variables and depressive symptoms, as well as between sleep-related variables
and medical errors, were similar for both PGY-1 and PGY-2 residents.

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Unadjusted
Duty hour categoryN

Odds ratio (95% CI)

Adjusted*
p-value

Odds ratio (95% CI)

p-valueSleep duration
C1: < 50 h7540.73 (0.59–0.91)

0.004

0.71 (0.59–0.91)

0.006

C2: ≥ 50 to < 60 h

1246

0.86 (0.72–1.03)

0.11

0.89 (0.73–1.08)

0.24

C3: ≥ 60 to < 70 h

1177

ReferenceC4: ≥ 70 to < 80 h

689

0.83 (0.67–1.03)

0.10

0.81 (0.64–1.02)

0.07

C5: ≥ 80 to < 90 h

7700.93 (0.76–1.14)

0.48

0.91 (0.73–1.14)

0.41

C6: ≥ 90 to < 100 h249

1.38 (1.03–1.84)

0.03

1.44 (1.06–1.97)

0.02

C7: ≥ 100 h

2101.58 (1.16–2.15)

0.004

1.30 (0.93–1.82)

0.13

Insomnia symptoms
C1: < 50 h

754

1.51 (1.26–1.80)< 0.001

1.58 (1.32–1.90)< 0.001

C2: ≥ 50 to < 60 h

1246

1.30 (1.12–1.51)

0.001

1.31 (1.13–1.53)< 0.001

C3: ≥ 60 to < 70 h

1177

ReferenceC4: ≥ 70 to < 80 h

689

0.89 (0.75–1.07)

0.21

0.91 (0.76–1.09)

0.31

C5: ≥ 80 to < 90 h

7700.86 (0.72–1.02)

0.09

0.90 (0.75–1.07)

0.22

C6: ≥ 90 to < 100 h249

0.60 (0.47–0.78)< 0.001

0.63 (0.49–0.82)

0.00

C7: ≥ 100 h

2100.53 (0.40–0.71)< 0.001

0.58 (0.44–0.78)< 0.001

Table 2.  Association between weekly duty hours and resident sleep-related variables analysed using
proportional odds regression models. The association between DHs and sleep durations, as well as between
DHs and insomnia symptoms were analysed using proportional odds regression models with generalized
estimating equations to account for hospital variability. The reference group for the analysis was duty hour
category 3 (≥ 60 to 70 h/week). Sleep duration was categorized into six categories per day: ≤ 4, 5, 6, 7, 8, and ≥ 9h. Insomnia symptoms was classified by scores on the Athens Insomnia Scale, with 0–5 points as “absence”, 6–9
points as “mild”, 10–15 points as “moderate”, and 16–24 points as “severe”. CI confidence interval. *Both models
are adjusted for age, sex, postgraduate year, hospital type, emergency department duty, assigned inpatient, selfstudy time, and depressive symptoms.

Unadjusted
Duty hour category

N

Prevalence ratio (95% CI)

Adjusted*
p-value

Prevalence ratio (95% CI)

p-valueSleep duration
<4 h

67

2.57 (2.01–3.28)< 0.001

2.12 (1.66–2.72)< 0.0015h

726

1.68 (1.47–1.92)< 0.001

1.56 (1.37–1.78)< 0.001

0.19

1.05 (0.94–1.18)

0.39

6h

2556

1.08 (0.96–1.22)

7h

1483

Reference8h

2321.13 (0.89–1.45)

0.31

1.11 (0.87–1.41)

0.42

>9 h

31

1.70 (1.03–2.78)

0.04

1.64 (0.97–2.76)

0.06

Insomnia ­symptoms†
Absence3737

Reference

Mild

1027

3.20 (2.87–3.56)< 0.001

3.10 (2.77–3.47)< 0.001

Moderate to severe331

5.12 (4.60–5.70)< 0.001

4.86 (4.35–5.42)< 0.001

Table 3.  Association between resident sleep and depressive symptoms analysed using modified Poisson
regression models. The association between sleep-related variables (sleep duration and insomnia symptoms)
and depressive symptoms were analysed using modified Poisson regression models with generalized estimating
equations to account for hospital variability. Depressive symptoms were measured dichotomously with
the Patient Health Questionnaire (PHQ)-2. CI confidence interval. *Adjusted for age, sex, postgraduate
year, hospital type, weekly duty hour, emergency department duty, assigned inpatient, and self-study time.
†
Insomnia symptoms was classified by scores on the Athens Insomnia Scale, with 0–5 points as “absence”, 6–9
points as “mild”, 10–15 points as “moderate”, and 16–24 points as “severe”. For the analysis, the “moderate” and“severe” categories were combined due to the small number of participants in the “severe” category (N = 45).

Path analysis

Figure 2 summarizes the results of the path analysis conducted using SEM to examine the associations among
weekly duty hours, sleep-related variables, depressive symptoms, and severe medical errors. Depressive symptoms were directly influenced by insomnia symptoms (β [95% CI]: 0.43 [0.41–0.46]) and sleep duration (0.03[0.001–0.05]), with insomnia symptoms having a greater effect. Similarly, medical errors were directly affected by
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Minor error
Unadjusted
Duty hour categoryN

Odds ratio (95% CI)

Adjusted*
p-value

Odds ratio (95% CI)

p-valueSleep duration
<4 h

67

1.09 (0.67–1.77)

0.72

1.00 (0.61–1.65)

1.00

5h

726

0.93 (0.78–1.11)

0.43

0.93 (0.77–1.11)

0.39

6h

2556

0.95 (0.84–1.08)

0.43

0.95 (0.84–1.08)

0.42

7h

1483

Reference8h

2321.07 (0.81–1.41)

0.63

1.13 (0.86–1.49)

0.38

>9 h

31

0.71 (0.33–1.51)

0.37

0.73 (0.34–1.57)

0.43

Absence3737

Reference

Mild

1027

1.28 (1.12–1.47)< 0.001

1.21 (1.05–1.40)

0.01

Moderate to severe

331

1.57 (1.26–1.96)< 0.0011.42 (1.13–1.81)

0.004

Unadjusted

Adjusted*

Odds ratio (95% CI)

p-value

Odds ratio(95% CI)

Insomnia ­symptoms†

Severe error

Duty hour categoryNp-valueSleep duration
<4 h

67

1.64 (0.92–2.93)

0.09

1.50 (0.83–2.70)

0.19

5h

726

0.86 (0.68–1.09)

0.22

0.82 (0.64–1.05)

0.10

0.22

0.90 (0.76–1.06)

0.21

6h

2556

0.90 (0.76–1.07)

7h

1483

Reference8h

2321.12 (0.79–1.59)

0.53

1.14 (0.80–1.62)

0.48

>9 h

31

2.69 (1.25–5.77)

0.01

2.70 (1.25–5.80)

0.01

Absence3737

Reference

Mild

1027

1.27 (1.06–1.52)

0.01

1.17 (0.97–1.42)

0.10

Moderate to severe

331

2.40 (1.82–3.05)< 0.001

1.92 (1.45–2.56)< 0.001

Insomnia ­symptoms†

Table 4.  Association between resident sleep-related variables and medical errors analysed using proportional
odds regression models. CI confidence interval. The association between sleep-related variables (sleep durationand insomnia symptoms) and medical errors were analysed using modified Poisson regression models withgeneralized estimating equations to account for hospital variability. The number of medical errors that resulted
in minor or severe disability or disadvantage to patients experienced in the past year were reported based on
the following numbers: 0, 1, 2, 3, 4, or 5 or more times. *Adjusted for age, sex, postgraduate year, hospital type,weekly duty hours, emergency department duty, assigned inpatient, self-study time, and depressive symptoms.
†
Insomnia symptoms was classified by scores on the Athens Insomnia Scale, with 0–5 points as “absence”, 6–9
points as “mild”, 10–15 points as “moderate”, and 16–24 points as “severe”. For the analysis, the “moderate” and“severe” categories were combined due to the small number of participants in the “severe” category (N = 45).

insomnia symptoms (0.10 [0.07–0.13]) and sleep duration (0.05 [0.02–0.07]), with insomnia symptoms having alarger effect. Weekly duty hours had an indirect effect on these effects (to insomnia symptoms: 0.08 [0.05–0.11],
to sleep duration: − 0.14 [− 0.17 to − 0.11]). In addition to this indirect effect, we also modeled a direct effectof duty hours on depressive symptoms and medical errors. However, the direct effect on depressive symptoms
was not statistically significant (0.01 [− 0.01 to 0.04]), while the effect on medical errors was significant (0.07[0.04–0.09]). Our model demonstrated a good fit to our data, as indicated by the root mean square error of
approximation (RMSEA) of 0.048, the comparative fit index (CFI) of 0.99, and the Tucker–Lewis index (TLI)
of 0.93. The result of another path analysis, which replaced severe medical error with minor medical error, ispresented in Supplementary Fig. 1. The results of the path analysis with minor errors were identical to thosewith severe medical errors.

Discussion

This cross-sectional study, conducted in Japan, included 5095 PGY-1 and PGY-2 residents and investigated theassociation between resident DH, sleep, and sleep-related outcomes. The results showed that 15.5% of the residents had chronic sleep deprivation (sleep duration < 5 h/day), and 26.7% had insomnia symptoms. Multivariable analysis indicated that DH > 90 h/week were associated with shorter sleep duration and increased severityof insomnia symptoms. However, sleep duration was prolonged when DH was < 60 h/week, and the severity of
insomnia symptoms decreased when DH was < 50 h/week. Shorter sleep durations (< 4 or 5 h/night) and more
severe insomnia symptoms were significantly associated with depressive symptoms, and insomnia symptoms
were linked to a greater number of medical errors, regardless of severity. Surprisingly, sleep duration did notaffect the number of errors.
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Figure 2.  Results of structural equation modeling among the association between weekly duty hours, sleeprelated variables, depressive symptoms, and severe medical error. Insomnia symptoms and sleep duration are
considered as covariates. The values shown are standardized profit regression coefficients. Solid lines indicate
statistical significance (p < 0.05), while dashed lines represent relationships that are not statistically significant.
Our model was well fitted to our data (root mean square error of approximation [RMSEA]: 0.048; comparative
fit index [CFI]: 0.99; Tucker-Lewis index [TLI]: 0.93).

This study revealed that > 90 h weekly DH was associated with reduced sleep duration and heightened insomnia symptoms among residents. This is an important discovery as Japan intends to restrict residents’ DH imminently in 2024, wherein the maximum weekly DH for residents is expected to be 80 h, which may improve sleep
quality. Furthermore, restricting DH to ≤ 60 h/week, as for general physicians, may increase sleep duration andalleviate insomnia symptoms. However, the disparity between the 80-h and 60-h weekly limit is not substantial.
Therefore, when determining the degree to which work hours should be reduced to enhance sleep quality, various
perspectives should be considered, including resident education, well-being, and patient care quality.
In this study, various factors that affect residents’ sleep were examined. First, the study discovered that the
number of additional ED duties in they did not affect resident sleep although the average number of inpatientsassigned to a resident at one time did. This implies that chronic sleep deprivation resulting from a demanding
daily workload may have a more significant effect on sleep than acute sleep deprivation resulting from long shifts.
Second, several specific resident characteristics, including older age, PGY-1 level, male sex, and university hospital affiliation, were associated with poor sleep quality. Third, the study showed that the duration of self-study
among residents was linked to shorter sleep duration; however, paradoxically, they also had fewer insomnia
symptoms. This may be because residents without insomnia can study for extended periods.
In this study, short sleep duration and insomnia symptoms were clearly related to depressive symptoms.
Approximately 20% of the patients with insomnia experience ­depression32. A recent large cohort study of US
medical interns found that longer weekly DH were associated with a greater increase in depressive symptoms
from ­baseline8. The rate of depression among resident physicians in Japan is comparable to healthcare professionals in ICUs during the COVID-19 pandemic, necessitating depression reduction ­measures33. Potentially,
reducing the number of hours worked per week may improve sleep and well-being of residents.
Notably, the number of medical errors experienced by residents was associated with insomnia symptoms
but not with sleep duration. A similar finding was reported in a 2019 study of general workers, where insomnia
symptoms, but not sleep duration, were linked to cognitive errors in the ­workplace34. This contrasts with several previous reports that found an association between sleep duration and cognitive ­errors35,36. For instance, aprospective study involving 1215 depression-free interns who slept < 6 h/night reported a higher rate of medical­errors36. The reason for this inconsistency might be attributed to variations in job responsibilities, work scope,and degrees of sleep deprivation. In this study, no significant difference was observed in the association between
medical errors and sleep duration among PGY-1 and PGY-2 residents. However, such an association might exist
among physicians who typically bear more responsibility in high-intensity clinical settings. Insomnia appears to
be particularly linked to an increase in medical errors, underscoring the importance of interventions to improve
sleep quality. Reducing the DH to < 90 h/week may mitigate the exacerbation of insomnia symptoms and reduce
medical errors. However, our data suggest that reducing DHs beyond 90 h/week does not make much of a difference regarding the presence or absence of insomnia. Therefore, it may be beneficial to consider additional
approaches to address insomnia beyond simply reducing DH.
This study recommends several ways to improve the working environments of residents in Japan. First, weekly
DH should be limited to < 90 h/week because working more has been shown to worsen chronic sleep deprivationand insomnia. Specifically, our data suggest that interventions targeting insomnia symptoms might be particularly
beneficial for PGY-1 residents. The upcoming implementation of a weekly DH limit of 80 h/week for residents
by 2024 aligns with this recommendation. However, as residents often express a willingness to exceed this limit,
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program leaders should provide adequate explanation and ­monitoring37. Second, reducing daily workloads (e.g.,assigned inpatients) may be more effective in improving sleep quality than reducing irregular duties (e.g., ED
duty shifts). Though some hospitals focus on reducing weekly DH by decreasing ED duties, this intervention
may not be sufficient to address sleep impairment. Third, to effectively reduce medical errors, it is necessary
to implement a multifaceted intervention targeting insomnia symptoms rather than simply maintaining sleep
duration. Clinical training hospital directors should identify residents at high risk for insomnia and monitor
their symptoms closely. Moreover, screening at entry may be useful, as a previous report indicates that 5–10%
of residents have insomnia symptoms at the start of t­ raining38.
This study had several limitations. First, all data on resident DH, sleep, medical safety, and depressive symptoms were self-reported and may have been subject to response bias. In particular, reports of medical errors may
be influenced by social desirability bias. To mitigate potential bias, we highlighted in the electronic questionnaire that the survey responses would be anonymous and would not impact the examination results. Second, it
was not possible to specify a causal relationship between each outcome and sleep status. Depressive symptoms
can cause sleep disturbances, and prolonged sleep deprivation can lead to depressive symptoms, which makes
it difficult to determine ­causality32. We interpreted the study results as an increase in medical errors based on
the presence or absence of insomnia symptoms. However, data on medical errors and insomnia symptoms werecollected at different times (within 1 year and 1 month, respectively). Therefore, to interpret these results, wemust assume that insomnia symptoms are chronic. Future follow-up studies are warranted to monitor resident
sleep and medical errors. Third, there is a possibility of selection bias. The sample of residents in this study was
limited to 18.2% of those affiliated with university hospitals, although university-affiliated residents compriseapproximately half of all residents in the country. This study indicated that university-affiliated residents had
shorter sleep durations than community hospital residents, which may have affected the overall results. Fourth,
the limited scope of the survey questions precluded a detailed understanding of the nature of the medical errors
experienced by the residents. Consequently, there is insufficient data to assess how improvements in resident
sleep might benefit patient care.

Conclusions

Working > 90 h/week was associated with short sleep duration and greater insomnia symptoms among residents.
Both short sleep duration and insomnia symptoms were associated with higher levels of depression amongresidents, whereas only insomnia symptoms were associated with increased medical errors. Thus, the 80-h
workweek limit in Japan may potentially improve residents’ sleep, improve well-being, and promote medical
safety. However, additional improvements could be achieved through sleep interventions that go beyond focusing solely on reducing work hours.

Data availability

The datasets generated during and/or analysed during the current study are not publicly available due to thenature of this research but are available from the corresponding author on reasonable request.
Received: 8 August 2023; Accepted: 3 January 2024

References

1. West, C. P., Dyrbye, L. N. & Shanafelt, T. D. Physician burnout: Contributors, consequences and solutions. J. Intern. Med. 283,
516–529 (2018).
2. Rothenberger, D. A. Physician burnout and well-being: A systematic review and framework for action. Dis. Colon Rectum 60,
567–576 (2017).
3. Kivimäki, M. et al. Long working hours and risk of coronary heart disease and stroke: A systematic review and meta-analysis of
published and unpublished data for 603,838 individuals. Lancet 386, 1739–1746 (2015).
4. Ministry of Health, Labour and Welfare. Addressing the reform of the working style of physicians in the clinical training system;(2023, accessed 2 Ma 2023). https://​www.​mhlw.​go.​jp/​conte​nt/​10803​000/​00059​0866.​pdf (in Japanese).
5. Nagasaki, K. & Kobayashi, H. The effects of resident work hours on well-being, performance, and education: A review from aJapanese perspective. J. Gen. Fam. Med. 24, 323–331 (2023).
6. Nagasaki, K. et al. Association between mental health and duty hours of postgraduate residents in Japan: A nationwide crosssectional study. Sci. Rep. 12, 10626 (2022).
7. Ogawa, R. et al. The relationship between long working hours and depression among first-year residents in Japan. BMC. Med.
Educ. 18, 50 (2018).
8. Fang, Y. et al. Work hours and depression in U.S. first-year physicians. N. Engl. J. Med. 387, 1522–1524 (2022).
9. Virtanen, M. et al. Long Working Hours and Sleep Disturbances: The whitehall II prospective cohort study. Sleep 32, 737–745(2009).
10. Taoda, K., Nakamura, K., Kitahara, T. & Nishiyama, K. Sleeping and working hours of residents at a national university hospitalin Japan. Ind. Health 46, 594–600 (2008).
11. Shea, J. A. et al. A randomized trial of a three-hour protected nap period in a medicine training program: Sleep, alertness, andpatient outcomes. Acad. Med. 89, 452–459 (2014).
12. Volpp, K. G. et al. Effect of a protected sleep period on hours slept during extended overnight in-hospital duty hours among medical
interns: A randomized trial. JAMA 308, 2208–2217 (2012).
13. Basner, M. et al. Sleep and alertness in a duty-hour flexibility trial in internal medicine. N. Engl. J. Med. 380, 915–923 (2019).
14. Van Dongen, H. P. et al. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions andsleep physiology from chronic sleep restriction and total sleep deprivation. Sleep 26, 117–126 (2003).
15. Gohar, A. et al. Working memory capacity is decreased in sleep-deprived internal medicine residents. J. Clin. Sleep Med. 5, 191–197(2009).
16. Williamson, A. M. & Feyer, A. M. Moderate sleep deprivation produces impairments in cognitive and motor performance equivalent to legally prescribed levels of alcohol intoxication. Occup. Environ. Med. 57, 649–655 (2000).

Scientific Reports |(2024) 14:1481 |https://doi.org/10.1038/s41598-024-51353-8

9Vol.:(0123456789)

www.nature.com/scientificreports/
17. Trockel, M. T. et al. Assessment of physician sleep and wellness, burnout, and clinically significant medical errors. JAMA Netw.
Open 3, e2028111 (2020).
18. Landrigan, C. P. et al. Effect of reducing interns’ work hours on serious medical errors in intensive care units. N. Engl. J. Med. 351,
1838–1848 (2004).
19. Landrigan, C. P. et al. Effect on patient safety of a resident physician schedule without 24-hour shifts. N. Engl. J. Med. 382,
2514–2523 (2020).
20. Bianchi, R., Schonfeld, I. S. & Laurent, E. Burnout-depression overlap: A review. Clin. Psychol. Rev. 36, 28–41 (2015).
21. McTaggart, L. S. & Walker, J. P. The relationship between resident physician burnout and its’ effects on patient care, professionalism, and academic achievement: A review of the literature. Heal. Sci. Rev. 4, 100049 (2022).
22. Tokuda, Y. et al. The interrelationships between working conditions, job satisfaction, burnout and mental health among hospitalphysicians in Japan: A path analysis. Ind. Health 47, 166–172 (2009).
23. Mustahsan, S. M. et al. Sleep deprivation and its consequences on house officers and postgraduate trainees. J. Pak. Med. Assoc. 63,
540–543 (2013).
24. Mendelsohn, D. et al. Impact of work hours and sleep on well-being and burnout for physicians-in-training: The resident activity
tracker evaluation study. Med. Educ. 53, 306–315 (2019).
25. Sephien, A., Reljic, T., Jordan, J., Prida, X. & Kumar, A. Resident duty hours and resident and patient outcomes: Systematic review
and meta-analysis. Med. Educ. 57, 221–232 (2023).
26. Imrie, K. R., Frank, J. R. & Parshuram, C. S. Resident duty hours: Past, present, and future. BMC Med. Educ. 14, 1–3 (2014).
27. Kozu, T. Medical education in Japan. Acad. Med. 81, 1069–1075 (2006).
28. Okajima, I., Nakajima, S., Kobayashi, M. & Inoue, Y. Development and validation of the Japanese version of the athens insomnia
scale. Psychiatry Clin. Neurosci. 67, 420–425 (2013).
29. Okajima, I. et al. Evaluation of severity levels of the Athens Insomnia scale based on the criterion of insomnia severity index. Int.
J. Environ. Res. Public Health 17, 8789 (2020).
30. Muramatsu, K. et al. The patient health questionnaire, Japanese version: validity according to the mini-international neuropsychiatric interview-plus. Psychol. Rep. 101, 952–960 (2007).
31. Manea, L. et al. Identifying depression with the PHQ-2: A diagnostic meta-analysis. J. Affect. Disord. 203, 382–395 (2016).
32. Tsuno, N., Besset, A. & Ritchie, K. Sleep and depression. J. Clin. Psychiatry 66, 1254–1269 (2005).
33. Kuriyama, A. et al. Burnout and turnover intention in critical care professionals during the COVID-19 pandemic in Japan: A
cross-sectional survey. Ann. Am. Thorac. Soc. 20, 262–268 (2023).
34. Brossoit, R. M. et al. The effects of sleep on workplace cognitive failure and safety. J. Occup. Heal. Psychol. 24, 411–422 (2019).
35. Lim, J. & Dinges, D. F. A meta-analysis of the impact of short-term sleep deprivation on cognitive variables. Psychol. Bull. 136,
375–389 (2010).
36. Kalmbach, D. A., Arnedt, J. T., Song, P. X., Guille, C. & Sen, S. Sleep disturbance and short sleep as risk factors for depression andperceived medical errors in first-year residents. Sleep https://​doi.​org/​10.​1093/​sleep/​zsw073 (2017).
37. Fletcher, K. E. et al. Why residents consider working beyond the duty hour limits: Implications of the ACGME 2011 duty hour
standards. J. Grad. Med. Educ. 3, 571–573 (2011).
38. Pulliam, S. J., Weinstein, D. F., Malhotra, A., Macklin, E. A. & Berkowitz, L. R. Baseline sleep dysfunction among matriculatinginterns. J. Grad. Med. Educ. 4, 202–208 (2012).

Acknowledgements

We thank members of the JAMEP, the question development committee, and the peer-review committee of theGeneral Medicine In-training Examination for their assistance. The authors would like to thank Editage (https://​www.​edita​ge.​jp) for English language review.

Author contributions

K.N.: conceptualization, writing—original draft, methodology, software, formal analysis, and visualization. H.K.:
conceptualization, methodology, writing-review and editing, supervision. Y.N.: conceptualization, investigation,
resources, writing-review and editing, supervision, and project administration. M.K.: conceptualization andinvestigation. T.W.: conceptualization, investigation, and writing-review and editing. T.S.: investigation. Y.Y.:
investigation. K.S.: investigation. S.F.: investigation. S.N.: Investigation. K.K.: investigation. Y.T.: conceptualization and supervision.

Competing interests

Dr. Nishizaki received an honorarium from JAMEP as the GM-ITE project manager. Dr. Tokuda is the JAMEP
director. Dr. Kobayashi and Dr. Shikino received an honorarium from JAMEP as speakers for the JAMEP lecture.
Dr. Shimizu, Dr. Yamamoto, Dr. Shikino, and Dr. Fukui received an honorarium from JAMEP as exam preparers
for GM-ITE. Dr. Nishizaki, Dr. Tokuda, Dr. Kobayashi, Dr. Shimizu, Dr. Yamamoto, Dr. Shikino, and Dr. Fukui
were not involved in this analysis. Dr. Nagasaki, Dr. Kurihara, Dr. Watari, Dr. Nishiguchi, and Dr. Katayama
have no conflicts of interest to declare.

Additional information

Supplementary Information The online version contains supplementary material available at https://​doi.​org/​
10.​1038/​s41598-​024-​51353-8.
Correspondence and requests for materials should be addressed to H.K.
Reprints and permissions information is available at www.nature.com/reprints.
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