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Keywords:

  • Japanese;
  • mortality;
  • sleep duration

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

Summary

Although several studies have examined the association between sleep duration and all-cause or cause-specific mortality, it is unclear whether long sleep duration might merely reflect decreased physical strength and poorer health status. We therefore examined the association between sleep duration and all-cause and cause-specific mortality, and conducted stratified analysis based on physical function and self-rated health. This study used prospective data from the Ohsaki Cohort Study, conducted in Miyagi Prefecture, in northern Japan. This study population comprised 49 256 subjects aged 40–79 years at the baseline survey. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) of all-cause and cause-specific mortality according to the five categories of sleep duration (≤6, 7, 8, 9, ≥10 h day−1), treating 7 h as the reference group, employing Cox's proportional hazard regression analysis. We found that long sleep duration was associated with mortality. The HRs (95% CIs) of subjects who slept more than 10 h were 1.37 (1.27–1.47), 1.49 (1.30–1.71) and 1.53 (1.36–1.73) for mortality due to all causes, total cardiovascular disease and other causes of death mortality, respectively. The association between long sleep duration and stroke mortality was especially marked among subjects with limited physical function and poorer health status. However, we did not observe such a trend for mortality due to all causes, total cardiovascular disease, ischaemic heart disease, cancer or other causes of death. We conclude that, with the exception of stroke mortality, the association between long sleep duration and mortality is not modified by physical function or health status.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

Sleep duration and mortality risk have been indicated in several previous studies (Cappuccio et al., 2010, 2011; Gallicchio and Kalesan, 2009). Although three meta-analyses have concluded that both long and short sleep duration were associated with an increased risk of mortality, the effect was larger among those who slept longer (long sleeper) than among short sleepers (Cappuccio et al., 2010, 2011; Gallicchio and Kalesan, 2009). The meta-analysis by Cappuccio et al., 2011 also reported that the hazard ratio (HR) of long sleep duration was higher than of short sleep with regard to mortality due to total cardiovascular disease (CVD) and stroke. The results of previous studies conducted in East Asia, predominantly Japan, have tended to indicate stronger HRs for both short and long sleep duration than those of studies conducted in Europe or the United States (Cappuccio et al., 2010). This difference might be attributable to 'differences in average sleep duration among countries, or to the longer life expectancy in Japan (Cappuccio et al., 2010; Steptoe et al., 2006).

However, it remains unclear whether long sleep duration directly increases the risk, or whether it merely reflects the presence of comorbidity or unhealthy status. In a cross-sectional study, Stranges et al., 2008 reported that long sleepers had lower physical activity and lower scores on the Short Form-36 physical scale. They suggested that long sleep duration was a reflection of comorbidity or poor health status. Conversely, Mesas et al., 2010 conducted a stratified analysis based on health status and physical function in a prospective cohort study. They found that long sleep duration was associated with greater mortality, irrespective of health status, and considered that the association between long sleep duration and mortality would not be explained by poorer health status among long sleepers. However, their sample size was small and they did not examine causes of death, even though some studies have suggested that the association between sleep duration and mortality risk differs according to cause of death (Amagai et al., 2004; Burazeri et al., 2003; Ferrie et al., 2007; Ikehara et al., 2009; Lan et al., 2007; Patel et al., 2004; Qureshi et al., 1997).

We therefore examined the association between sleep duration and all-cause and cause-specific mortality, and conducted stratified analysis by physical function and self-rated health.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

Study cohort

We used data obtained from the Ohsaki National Health Insurance (NHI) Cohort Study, details of which have been described elsewhere (Kuriyama et al., 2006; Tsuji et al., 1998). Briefly, we delivered a self-administered questionnaire, including items on sleep duration, between October and December 1994 to all NHI beneficiaries aged 40–79 years living in the catchment area of Ohsaki Public Health Center, a local government agency that provides preventive health services for residents of 14 municipalities in Miyagi Prefecture, northern Japan. Of 54 996 eligible men (26 481) and women (28 515), 52 029 (94.6%) responded (men: 24 895, women: 27 134).

To ascertain the date of, and reason for, withdrawal from the NHI, we began prospective collection of NHI withdrawal history files on 1 January 1995. We excluded 776 participants who had withdrawn from the NHI before the baseline questionnaire survey. Thus, 51 253 participants (men: 24 573, women: 26 680) ultimately formed the study cohort.

Exposure measurement

The questionnaire included items about sleep duration, as well as alcohol drinking and smoking habits, a 40-item food frequency questionnaire (FFQ), personal and family history of diseases, job status, level of education, marital status, body weight, height, time spent walking, physical function, self-rated health and perceived mental stress.

For items related to sleep duration, participants entered the mean integer number of hours of sleep they had taken per day during the last year. We categorized sleep duration into five groups: ≤6, 7, 8, 9 and ≥10 h day−1. We rounded-off sleep duration to the closest whole number.

The physical function status of each subject was assessed using the self-completed questionnaires returned at the baseline survey in 1994 using the six-item physical function measure of the Medical Outcomes Study (MOS) Short-form General Health Survey (Stewart et al., 1988, 1989; Tsuji et al., 1999; Ware et al., 1996). This measure examines the extent to which health affects a variety of physical activities, ranging from strenuous exercise to basic self-care. The validity and reliability of the MOS questionnaire have been fully established (Stewart et al., 1988, 1989; Ware et al., 1996). The Japanese version of the MOS scale has been reported to predict all-cause mortality, hospitalization risk and medical costs (Tsuji et al., 1999). In the analysis, we classified the subjects into the following seven groups according to their self-response, which was referred to as the MOS score: level 6, able to perform vigorous activities such as lifting heavy objects, running or participating in strenuous sports; level 5, able to perform moderate activities such as moving a table, carrying groceries or bowling; level 4, able to walk uphill or climb a few flights of stairs; level 3, able to bend, lift or stoop; level 2, able to walk one block; level 1, able to perform self-care activities such as eating, dressing, bathing or using the toilet; level 0, unable to do anything unaided (Tsuji et al., 1999). This classification was ordered hierarchically in terms of difficulty in performing physical tasks, and we scored the levels of each subject according to the highest physical task he/she answered as being not limited at all. For each item measured, we classified the subjects into two groups: ‘limited’ (levels 0–4) and ‘unlimited’ (levels 5 and 6).

Self-rated health was assessed through the subject's response to the question: ‘How is your overall health status?’. The subjects were asked to choose one of five answers: ‘bad’, ‘poor’, ‘moderate’, ‘good’ or ‘excellent’, and on the basis of their responses, we classified them into two groups: ‘worse’ (poor or bad) and ‘better’ (excellent, good or moderate).

Follow-up

The end-points were mortality due to all causes, CVD [ischaemic heart disease (IHD) and stroke], cancer and other causes. We followed-up the subjects for mortality and emigration by reviewing the NHI withdrawal history files from 1 January 1995 to 31 March 2008. When a subject withdrew from the NHI system because of death, emigration or employment, the date of withdrawal and the reason were coded on the NHI withdrawal history files. Because we were unable to obtain subsequent information for subjects who withdrew from the NHI, we discontinued their follow-up. For deaths thus identified, we investigated the causes by reviewing the death certificates filed at Ohsaki Public Health Center. Cause of death was coded by trained physicians according to the International Classification of Diseases and Related Health Problems, Tenth Revision (ICD-10) (WHO, 1992). We identified deaths due to CVD as codes I00–I99 (including IHD as codes I20–I25 and strokes as codes I60–I69), and those due to cancer as codes C00–C97.

After exclusion of subjects who had not entered responses for sleep duration (= 1 783), and who had reported a sleep duration of <4 h or more than 12 h (= 214), 49 256 participants (men: 23 749, women: 25 507) remained, including 8447 participants who had died due to all causes (CVD 2549; cancer 2764; other causes 3134). In order to improve the reliability of our questionnaire, we excluded participants who had reported sleep durations of <4 h or more than 14 h, because these durations were considered to be extremely long or short. Also, these subjects were less likely to have answered the question items (data not shown).

Ethical permissions

The study protocol was reviewed and approved by the ethics committee of Tohoku University School of Medicine. We considered the return of self-administered questionnaires signed by the subjects to imply their consent to participate in the study.

Statistical analysis

We counted person-years of follow-up for each of the subjects from 1 January 1995 until the date of death, the date of withdrawal from the NHI or the end of follow-up (31 March 2008), whichever occurred first. The average follow-up time was 10.8 years. The validity of the proportional hazards assumption was verified by adding a time-dependent variable to each model to confirm that the HR for each covariate did not increase or decrease over time.

We used Cox's proportional hazard regression analysis to estimate hazard ratios (HRs) and the 95% confidence intervals (CIs) of mortality according to the five categories of sleep duration (≤6, 7, 8, 9 and ≥10 h day−1), treating 7 h as the reference group. We chose 7 h as a reference group because this category had been used as a reference group in most previous studies, and we had predicted that this group would have the lowest mortality risk (Cappuccio et al., 2010, 2011; Gallicchio and Kalesan, 2009). In these analyses, we considered the following parameters as covariates: age (continuous variable); sex; total caloric intake (continuous variable, calculated from the 40-item FFQ); body mass index (BMI) in kg m−2 (<18.5, 18.5–24.9, ≥25.0); marital status (married or unmarried); level of education (junior high school or less, high school or college/university or higher); job status (employed or unemployed); history of myocardial infarction; history of cancer; history of stroke; history of hypertension; history of diabetes mellitus; smoking status (never smoker, ex-smoker, current smoker one to 19 cigarettes day−1, or current smoker ≥ 20 cigarettes day−1); alcohol drinking (never drinkers, ex-drinkers, current drinkers < 27.8 g day−1, current drinkers 27.8–45.59 day−1, current drinkers 45.6–68.39 day−1 or current drinkers ≥ 68.4 day−1 ethanol); time spent walking (<1 h day−1, 1 h day−1 or longer); perceived mental stress (low, moderate or high); self-rated health (worse or better); physical function (limited or unlimited). All the covariates we selected had been suggested to show an association with all-cause mortality or cause-specific mortality in the previous studies. We did not use a stepwise procedure. In addition, we conducted analyses stratified by physical function and self-rated health. In the stratified analyses, we excluded subjects for whom answers about health status and/or physical function were missing. Interactions between the five categories of sleep duration and physical function and self-rated health were tested for using the likelihood ratio test, which compared the models with and without cross-product interaction terms.

All statistical analyses were performed using the sas statistical software package, version 9.2 (SAS Institute Inc., Cary, NC, USA). All the statistical tests reported were two-sided. Differences at < 0.05 were accepted as statistically significant.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

Table 1 shows the baseline characteristics, according to sleep duration, separately for both sexes. For both men and women subjects, those who slept for 10 h or more per day were more likely to be older, to have a lower daily total caloric intake, to have a history of myocardial infarction, cancer, stroke, hypertension or diabetes mellitus, to have lower perceived mental stress, to have worse self-rated health and more limited physical function and were less likely to be employed or married (P-values < 0.0001). For both men and women, subjects those who slept 6 h or less per day were younger, less likely to have a history of myocardial infarction, cancer, stroke and hypertension and less limited physical function, and were more likely to have a higher education level and perceived mental stress (P-values < 0.0001).

image

Figure 1. Sleep duration and all-cause mortality. Hazard ratio and 95% confidence intervals according to sleep duration by all subjects, subjects who had limited physical function or subjects who had unlimited physical function.

Download figure to PowerPoint

Table 1. Baseline characteristics of the subjects according to sleep duration exclude extreme sleep duration
 Sleep duration (hours per day)P valuesa
≤678910
  1. a

    Continuous variables were analyzed by anova, and categorical variables were analyzed by chi-square test.

  2. SD, standard deviation. MI, myocardial infarction; DM, diabetes mellitus.

Men
Number of subjects2 8376 1609 8482 7232 181 
Mean age (years), SD58.2 (11.0)56.8 (10.5)59.5 (10.3)62.9 (9.4)66.0 (8.7)<0.0001
Mean Body Mass Index (kg m−2), SD23.6 (3.2)23.4 (2.9)23.3 (3.0)23.1 (3.2)22.9 (3.7)<0.0001
Mean total caloric intake (kcal day−1), SD1 735.1 (647.8)1 829.2 (629.4)1 806.3 (640.8)1 765.7 (641.9)1 649.9 (651.2)<0.0001
Having history of MI3.72.63.23.25.5<0.0001
Having history of cancer3.32.52.73.14.6<0.0001
Having hisotry of stroke2.41.42.63.98.6<0.0001
Having history of hypertension (%)23.620.824.729.131.2<0.0001
Having history of DM (%)9.06.67.27.59.5<0.0001
Employed (%)58.162.758.151.441.2<0.0001
Married (%)86.889.789.390.486.5<0.0001
Junior high school graduated or less50.052.160.069.772.5<0.0001
Never drinkers (%)15.216.316.314.615.4<0.0001
Never smokers19.018.617.315.615.9<0.0001
Time spent walking 1 h day−1 or longer (%)42.245.445.245.040.2<0.0001
High perceived mental stress (%)20.715.311.711.211.7<0.0001
Poor or bad self-rated health (%)20.414.717.218.231.6<0.0001
Limited physical function (%)18.012.616.521.036.8<0.0001
Women
Number of subjects4 8407 4869 1992 2591 723 
Mean age (years), SD59.1 (10.5)58.5 (10.0)61.6 (9.3)65.0 (8.4)68.5 (8.6)<0.0001
Mean Body Mass Index (kg m−2), SD23.6 (3.4)23.7 (3.3)23.8 (3.4)24.0 (3.5)23.9 (4.4)<0.0001
Mean total caloric intake (kcal day−1), SD1 216.6 (378.6)1 260.9 (367.5)1 238.5 (380.5)1 197.3 (402.2)1 127.4 (419.9)<0.0001
Having history of MI2.41.82.23.14.4<0.0001
Having history of cancer4.03.13.84.16.2<0.0001
Having hisotry of stroke1.20.91.62.56.2<0.0001
Having history of hypertension (%)25.325.229.635.239.3<0.0001
Having history of DM (%)5.54.85.77.410.4<0.0001
Employed (%)34.438.232.126.418.3<0.0001
Married (%)73.178.676.970.660.3<0.0001
Junior high school graduated orless47.147.157.865.966.3<0.0001
Never drinkers (%)54.860.260.460.858.7<0.0001
Never smokers69.071.968.866.863.0<0.0001
Time spent walking 1 h day−1 or longer (%)37.839.538.538.031.4<0.0001
High perceived mental stress (%)25.318.713.512.211.1<0.0001
Poor or bad self-rated health (%)24.318.722.226.938.8<0.0001
Limited physical function (%)29.225.232.943.456.0<0.0001

Table 2 shows the age- and sex-adjusted and multivariate adjusted HRs for all-cause and cause-specific mortality according to sleep duration. Among subjects who slept 10 h or more per day, there was a significantly increased risk of mortality due to all causes, total CVD, IHD, stroke and other causes, respectively. Among subjects who slept 6 h or less per day, there was a significantly increased IHD mortality, but no significant association with risk of mortality due to all causes, total CVD, stroke or other causes. In addition, all-cause, total CVD and IHD mortality risk were increased significantly among subjects who slept for 8 h and 9 h day−1. Mortality risk due to stroke and other causes of death was increased significantly among subjects who slept 9 h day−1.

Table 2. Cox proportional hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause and cause-specific mortality according to sleep duration
 Sleep duration (hours per day)
≤6789≥10
  1. a

    Multivariable HR was adjusted for age (continuous variable); sex; total caloric intake (continuous variable); body mass index in kg m−2 (<18.5, 18.5–24.9, ≥25.0); marital status (married or unmarried); level of education (junior high school or less, high school, or college/university or higher); job status (employed or unemployed); history of myocardial infarction; history of cancer; history of stroke; history of hypertension; history of diabetes mellitus; smoking status (never smoker, ex-smoker, current smoker 1–19 cigarettes day−1, or current smoker ≥ 20 cigarettes day−1); alcohol drinking (never drinkers, ex-drinkers, current drinkers < 27.8 g day−1, current drinkers 27.8–45.59 g day−1, current drinkers 45.6–68.39, current drinkers ≥ 68.4 g day−1 ethanol); time spent walking (<1 h day−1 or 1 h day−1 or longer); perceived mental stress (low, moderate, or high); self-rated health (worse or better), physical function (limited or unlimited). CVD, cardiovascular disease; IHD, ischaemic heart disease.

Person-years83 530150 684206 53752 15737 066
All-cause
No. of deaths1 0741 6713 2061 1171 379
Age- and sex-adjusted HR (95% CI)1.09 (1.01–1.17)1.00 (reference)1.10 (1.04–1.17)1.18 (1.09–1.27)1.63 (1.52–1.75)
Multivariable HR1 (95% CI)*1.01 (0.93–1.09)1.00 (reference)1.07 (1.01–1.14)1.14 (1.06–1.24)1.37 (1.27–1.47)
All CVD
No. of deaths325439972361452
Age- and sex-adjusted HR (95% CI)1.20 (1.04–1.39)1.00 (reference)1.26 (1.12–1.41)1.39 (1.21–1.60)1.88 (1.64–2.15)
Multivariable HR1 (95% CI)*1.10 (0.96–1.28)1.00 (reference)1.21 (1.08–1.36)1.32 (1.15–1.52)1.49 (1.30–1.71)
IHD
No. of deaths81912248283
Age- and sex-adjusted HR (95% CI)1.49 (1.10–2.01)1.00 (reference)1.40 (1.10–1.79)1.55 (1.15–2.10)1.73 (1.28–2.35)
Multivariable HR1 (95% CI)*1.38 (1.02–1.86)1.00 (reference)1.36 (1.06–1.73)1.49 (1.10–2.02)1.41 (1.04–1.92)
Stroke
No. of deaths143203435166218
Age- and sex-adjusted HR (95% CI)1.14 (0.92–1.41)1.00 (reference)1.23 (1.04–1.45)1.40 (1.14–1.73)1.99 (1.63–2.42)
Multivariable HR1 (95% CI)*1.05 (0.84–1.30)1.00 (reference)1.17 (0.99–1.39)1.30 (1.06–1.60)1.51 (1.24–1.85)
Cancer
No. of deaths3666371,071335355
Age- and sex-adjusted HR (95% CI)1.01 (0.89–1.15)1.00 (reference)0.99 (0.89–1.09)0.98 (0.85–1.12)1.20 (1.05–1.37)
Multivariable HR1 (95% CI)*0.97 (0.85–1.11)1.00 (reference)0.97 (0.88–1.07)0.96 (0.84–1.10)1.10 (0.96–1.25)
Other
No. of deaths3835951,163421572
Age- and sex-adjusted HR (95% CI)1.08 (0.95–1.23)1.00 (reference)1.11 (1.01–1.23)1.22 (1.08–1.38)1.86 (1.66–2.09)
Multivariable HR1 (95% CI)a0.98 (0.86–1.11)1.00 (reference)1.09 (0.99–1.20)1.20 (1.06–1.36)1.53 (1.36–1.73)

A significant interaction between sleep duration and physical function was observed only for stroke mortality (Table 3, Fig. 1). The association between long sleep duration and risk of stroke mortality was stronger among subjects who had limited physical function than among those who had unlimited physical function. Otherwise, there were no differences in the risk of mortality due to all causes, total CVD, IHD, cancer or other causes, irrespective of whether or not subjects had limited physical function.

Table 3. Cox proportional hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause and cause-specific mortality stratified by physical function status
 Number of deathsSleep duration (hours per day)P for interaction
≤6789≥10
  1. Adjusted for age (continuous variable); sex; total caloric intake (continuous variable); body mass index in kg m−2(<18.5, 18.5–24.9, ≥25.0); marital status (married or unmarried); level of education (junior high school or less, high school, or college/university or higher); job status (employed or unemployed); history of myocardial infarction; history of cancer; history of stroke; history of hypertension; history of diabetes mellitus; smoking status (never smoker, ex-smoker, current smoker 1–19 cigarettes day−1 or current smoker ≥ 20 cigarettes day−1); alcohol drinking (never drinkers, ex-drinkers, current drinkers < 27.8 g day−1, current drinkers 27.8–45.59 g day−1, current drinkers 45.6–68.39, current drinkers ≥ 68.4 g day−1 ethanol); time spent walking (<1 h day−1, or 1 h day−1 or longer); perceived mental stress (low, moderate, or high); self-rated health (worse or better).CVD, cardiovascular disease; IHD, ischaemic heart disease.

All-cause
Limited3 3621.08 (0.96–1.22)1.00 (reference)1.10 (1.00–1.21)1.24 (1.10–1.39)1.41 (1.26–1.57)0.15
Unlimited4 3170.94 (0.85–1.05)1.00 (reference)1.04 (0.96–1.12)1.04 (0.93–1.15)1.23 (1.10–1.38)
All-CVD
Limited1 2781.06 (0.86–1.32)1.00 (reference)1.20 (1.01–1.42)1.43 (1.17–1.75)1.58 (1.31–1.91)0.20
Unlimited1 1311.15 (0.93–1.41)1.00 (reference)1.24 (1.06–1.46)1.24 (1.00–1.52)1.21 (0.96–1.52)
IHD
Limited2631.31 (0.83–2.06)1.00 (reference)1.16 (0.79–1.69)1.70 (1.10–2.61)1.48 (0.97–2.26)0.19
Unlimited2691.52 (0.99–2.32)1.00 (reference)1.63 (1.16–2.29)1.25 (0.78–1.99)1.38 (0.83–2.28)
Stroke
Limited5941.20 (0.87–1.65)1.00 (reference)1.30 (1.00–1.69)1.57 (1.16–2.14)1.88 (1.42–2.49)0.04
Unlimited5010.93 (0.68–1.28)1.00 (reference)1.12 (0.89–1.41)1.15 (0.85–1.56)0.93 (0.65–1.34)
Cancer
Limited9111.11 (0.89–1.38)1.00 (reference)0.95 (0.79–1.14)0.96 (0.76–1.22)1.00 (0.80–1.25)0.07
Unlimited1 7180.86 (0.73–1.02)1.00 (reference)0.95 (0.84–1.07)0.92 (0.77–1.09)1.15 (0.96–1.38)
Other
Limited1 4761.08 (0.89–1.31)1.00 (reference)1.14 (0.97–1.33)1.29 (1.07–1.57)1.56 (1.32–1.86)0.41
Unlimited1 4680.90 (0.75–1.08)1.00 (reference)1.03 (0.90–1.17)1.06 (0.88–1.27)1.37 (1.13–1.66)

Table 4 shows the analysis stratified by self-rated health (worse or better). The association between long sleep duration and stroke mortality was stronger among subjects who reported worse health status than among subjects who reported better health status. In addition, the HR for short sleep duration among subjects who reported worse health status was higher than that for subjects who reported better health status. Otherwise, the risk of mortality due to all causes, total CVD, IHD, cancer and other causes showed no differences among the subjects, irrespective of health status.

Table 4. Cox proportional hazard ratios (HRs) and 95% confidence intervals (CIs) for all-cause and cause-specific mortality stratified by self-rated health
 Number of deathsSleep duration (hours per day)P for interaction
≤6789≥10
  1. Adjusted for age (continuous variable); sex; total caloric intake (continuous variable); body mass index (BMI) in kg m−2 (<18.5, 18.5–24.9, ≥25.0); marital status (married or unmarried); level of education (junior high school or less, high school, or college/university or higher); job status (employed, or unemployed); history of myocardial infarction; history of cancer; history of stroke; history of hypertension; history of diabetes mellitus; smoking status (never smoker, ex-smoker, current smoker 1–19 cigarettes day−1 or current smoker ≥ 20 cigarettes day−1); alcohol drinking (never drinkers, ex-drinkers, current drinkers < 27.8 g day−1, current drinkers 27.8–45.59 g day−1, current drinkers 45.6–68.39 or current drinkers ≥ 68.4 g day−1 ethanol); time spent walking (<1 h day−1, or 1 h day−1, or longer); perceived mental stress (low, moderate or high); physical function (limited or unlimited). CVD, cardiovascular disease; IHD, ischaemic heart disease.

All-cause
Worse (poor or bad)3 0111.00 (0.88–1.14)1.00 (reference)1.05 (0.95–1.17)1.18 (1.03–1.35)1.39 (1.23–1.57)0.61
Better (excellent to fair)5 1480.99 (0.90–1.09)1.00 (reference)1.06 (0.99–1.14)1.07 (0.97–1.18)1.26 (1.14–1.40)
All-CVD
Worse (poor or bad)9961.18 (0.93–1.48)1.00 (reference)1.23 (1.02–1.50)1.32 (1.04–1.68)1.59 (1.28–1.97)0.86
Better (excellent to fair)1 4631.05 (0.87–1.27)1.00 (reference)1.21 (1.05–1.40)1.30 (1.08–1.55)1.33 (1.10–1.61)
IHD
Worse (poor or bad)2121.04 (0.65–1.68)1.00 (reference)1.10 (0.75–1.63)1.29 (0.79–2.10)1.11 (0.70–1.78)0.77
Better (excellent to fair)3331.70 (1.14–2.53)1.00 (reference)1.64 (1.18–2.26)1.57 (1.05–2.34)1.76 (1.15–2.68)
Stroke
Worse (poor or bad)4521.45 (1.02–2.06)1.00 (reference)1.27 (0.94–1.73)1.49 (1.03–2.16)2.04 (1.48–2.82)0.046
Better (excellent to fair)6690.81 (0.60–1.08)1.00 (reference)1.12 (0.91–1.38)1.21 (0.93–1.56)1.06 (0.79–1.41)
Cancer
Worse (poor or bad)7840.93 (0.73–1.18)1.00 (reference)0.88 (0.72–1.07)1.02 (0.79–1.32)1.10 (0.87–1.39)0.60
Better (excellent to fair)1 8890.96 (0.82–1.12)1.00 (reference)0.97 (0.86–1.09)0.88 (0.75–1.03)1.06 (0.89–1.25)
Other
Worse (poor or bad)1 2310.94 (0.77–1.16)1.00 (reference)1.06 (0.90–1.26)1.21 (0.98–1.49)1.47 (1.22–1.77)0.34
Better (excellent to fair)1 7960.99 (0.83–1.17)1.00 (reference)1.07 (0.94–1.21)1.12 (0.95–1.31)1.46 (1.23–1.72)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

In this study, we found that long sleep duration was associated with an increased risk of mortality due to all causes, total CVD, IHD, stroke and other causes, and that short sleep duration was associated with an increased risk of IHD mortality. Significant interactions were observed only for stroke mortality, the risk of which was elevated among subjects who had limited physical function or worse health status. Otherwise, the risk of mortality due to all causes, total CVD, IHD, cancer and other causes was not increased among subjects who had limited physical function or worse health status.

The present results are consistent with most previous studies, including three meta-analyses of the association between longer sleep duration and all-cause mortality, CVD mortality or CVD incidence (Cappuccio et al., 2010, 2011; Gallicchio and Kalesan, 2009). In this study we have demonstrated no significant interaction between sleep duration and physical function or health status in terms of all-cause mortality, consistent with the results of Mesas et al., 2010, who found that long sleepers had an increased risk of mortality irrespective of health status. We also demonstrated that stroke mortality was increased significantly among subjects who had limited physical function, but not among subjects whose physical function was not restricted. We also found that mortality due to causes other than stroke, such as total CVD, IHD, cancer and other causes, showed no significant interaction with sleep duration, physical function or health status. In the present study, interactions between long sleep duration and physical function or health status were elevated only for stroke mortality, suggesting that the mechanism responsible for the association between long sleep duration and mortality differed among causes of death. However, Mesas et al. did not conduct any analysis of cause-specific mortality.

Several researchers have hypothesized that long sleep duration might reflect sleep need, reflecting in turn decreased physical strength, poor health status or accompanying comorbidity (Chen et al., 2008; Ikehara et al., 2009; Patel, 2009; Patel et al., 2004). Because long sleepers are more likely to have low physical activity and low scores on the Short Form-36 physical score scale (Stranges et al., 2008), it has been hypothesized that long sleep duration might be a consequence of unrecognized chronic comorbidity. We also conducted analysis stratified by history of diseases (cancer, myocardial infarction or stroke) that would provide an objective measure of health; however, this made no appreciable difference to the result (data not shown). Therefore, with the exception of stroke mortality, our results did not support this hypothesis in terms of mortality due to all causes, total CVD, IHD, cancer and other causes.

The present study is the first large-scale epidemiological study to have examined whether or not physical function or self-rated health modifies the association between sleep duration and all-cause and cause-specific mortality. Furthermore, we recruited subjects from a large general population, allowing possible generalization of our results. We followed a large number of participants over a 14-year period; as the response rate was 94.6%, the subjects were highly representative of the target population.

Our study also had several limitations. First, sleep duration was determined on the basis of a self-reported questionnaire, and the assessment was conducted only once. Lauderdale et al., 2008 reported that the correlation between self-reported sleep duration and that measured objectively by wrist actigraphy was moderate and systematically biased. There is a possibility that some misclassification occurred in our analysis; therefore, some misclassification of sleep duration could have arisen, and this might have affected its perceived association with mortality. However, any such misclassification would have been non-differential, and thus might have led to underestimation of the impact of sleep duration. Secondly, we had no information about sleep quality, the timing of sleep, presence of sleeping disorders, the use of sleep medication or other types of medication, such as antidepressants or benzodiazepines Such factors would have an influence on sleep duration, and thereby might affect the association between long sleep duration and mortality (Suzuki et al., 2009). Thus, considering the influence of such factors, our results might have been overestimated. Finally, we had no information about rotating shift work or night work, even though shift work might affect the association between sleep duration and CVD (Fujino, 2007). However, as about 15% of our study subjects were housewives, 30% were farmers and 20% were retirees, we consider that very few would have been involved in rotating or night shift work.

In conclusion, our study indicates that longer sleep duration appears to be associated with mortality due to all causes, total CVD, IHD and other causes, except for stroke mortality. Such an association was observed irrespective of physical function or self-rated health. The HR for stroke mortality in particular was markedly higher in subjects whose physical function was limited, or in those who had worse health status. Future studies of the association between longer sleep duration and cause-specific mortality may need to consider the effect of physical function and self-rated health.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References

This study was supported by a Health Sciences Research Grant for Health Services (H21-Choju-Ippan-001, H20-Junkankitou (Seisyu)-Ippan-013), H23-Junkankitou (Seisyu)-Ippan-005, Ministry of Health, Labour and Welfare, Japan.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. Disclosure statement
  9. References
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