To examine the relationship between short sleep duration and body mass index (BMI), waist circumference (WC), visceral fat area (VFA) and subcutaneous fat area (SFA) among a working population in Japan.
Health-center-based, cross-sectional study.
The study subjects included 5400 men and 642 women aged 30 to 75 years who underwent an abdominal computed tomography (CT) scanning examination in a comprehensive health checkup.
Height and weight were measured, and BMI was calculated. WC, VFA and SFA were measured using a CT scanner. Sleep duration was self-reported. Analysis of covariance was used to estimate adjusted means of BMI, WC, VFA and SFA across categories of sleep duration with adjustments for potential confounders. Trend of the association was assessed using multiple linear regression analysis.
In men, the mean values of BMI, WC and SFA decreased with increasing sleep duration after adjustment for age, physical activity, smoking and drinking (P-value for trend <0.001). Additional adjustment for physical illnesses did not attenuate the explanatory power of the models (P-value for trend <0.001). In addition, the association between sleep duration and SFA did not change after controlling for VFA (P-value for trend <0.001). The mean values of SFA for subjects sleeping ‘<5 h’, ‘5 to <6 h’, ‘6 to <7 h’ and ‘⩾7 h’ per day were 145.8±67.4 cm2, 138.7±61.5 cm2, 134.7±60.4 cm2 and 132.5±49.2 cm2, respectively. Sleep duration was not appreciably associated with VFA. In women, no significant association was detected in any models.
Shorter sleep duration is associated with higher BMI, WC and SFA in men. Further research is needed to explicate the biological mechanisms behind these relationships and to see whether interventions addressing inadequate sleep could treat or prevent obesity by taking gender differences into consideration.
In the past few decades, there has been a significant increase in the prevalence of obesity worldwide. The World Health Organization describes obesity as one of the most visible, yet neglected, public health problems, which threatens to overwhelm both more and less developed countries.1 The observation that obesity prevalence has increased over the past decades at the same time as sleep duration has decreased has drawn attention to the possibility that sleep deprivation may have contributed to the obesity epidemic.2 The interest may have been further promoted by the recognition that obesity epidemic cannot easily be attributed to the so-called obesogenic environment, which is assumed to lead to overeating and sedentary lifestyle.3
In Japan, the prevalence of obesity in men aged 30 to 60 years increased from ∼20% in 1986 to >30% in 2006.4 Meanwhile, the average sleep duration among Japanese has decreased steadily over the past 40 years. The average daily sleep duration in people aged 10 years and older has declined from 493 min (8.2 h) in 1960 to 447 min (7.5 h) in 1995, and further drop was observed in 2005 to 442 min (7.3 h).5 The small decrease in sleep duration from 1995 to 2005 may suggest that the average sleep in Japanese has reached the minimum requirement for human survival.6
Evidence has grown over the past decade supporting the roles of habitual sleep duration as a novel risk factor for obesity in both children and adults7 and its subsequent health outcomes including all-cause mortality,8 type 2 diabetes,9 hypertension10 and other cardiovascular outcomes.11 The relationship is typically a U-shaped curve where the lowest risk is found at ∼7 to 8 h of sleep per day with the odds rising for shorter and longer sleepers.12 This pattern suggests that different mechanisms may operate at either end of the distribution of sleep duration.13 As a result of these findings, both sleep restriction14 and sleep extension15 have been suggested for potential health interventions.
Although consistent findings have been observed in children, controversy remains in the relationship between sleep duration and obesity and/or weight gain in adults.2, 16 Such relationships have not been found in several epidemiological studies.17, 18 Furthermore, gender differences have also been reported, with the significant association found in men but not in women19 or vice versa.20 The interpretation and conclusion of the findings is hampered by fundamental conceptual and methodological issues.2, 16 Few studies, for example, have considered physical and mental health conditions as potential confounders. As shown in a recent German study, the relationship between sleep duration and body mass index (BMI) did not persist after controlling for self-rated physical health and emotional status.21
Moreover, the inconsistency might have been caused in part by the different definitions and measurements of obesity. BMI, an indicator of overall obesity, has been frequently used in previous studies to define weight gain and obesity. The importance of central obesity and abdominal fat mass has been known to have a stronger relation to the prevalence of each component of metabolic syndrome (hyperglycemia, diabetes and hypertension) than BMI.22 Our previous study has indicated a superior performance of visceral fat area (VFA) to predict the clustering of metabolic risk factors compared with BMI, waist circumference (WC) and subcutaneous fat area (SFA) in a Japanese population.23 It remains unclear, however, whether sleep duration is related to abdominal fat areas after taking potential confounders into account. The objective of this study was therefore to examine the relationship between sleep duration and BMI, WC, VFA and SFA in a large sample of Japanese working population enrolled in the Hitachi Health Study.
Materials and methods
Study procedure and subjects
This cross-sectional study was conducted in 2009 and 2010 during a comprehensive annual health examination conducted at the Hitachi Health Care Center, Ibaraki prefecture, Japan. The procedure of the study has been described elsewhere.24, 25 In brief, participants were asked to fill in a computer-based survey questionnaire on the day of the regular health checkup. In total, 17 606 male employees and their spouses underwent the checkup after having fasted overnight. Of these participants, 6537 subjects received an abdominal computed tomography (CT) scanning examination and were the targets for this study. We excluded subjects with a history of diabetes mellitus (n=229), stroke (n=28), myocardial infarction (n=39), cancer (n=38), psychiatric illnesses (n=98) and insomnia (n=59). We further excluded four subjects who did not provide information regarding sleep duration. Some subjects were overlapped and fell into more than one exclusion criteria. We finally included 6271 subjects in the analyses. We obtained written informed consent from each participant after the nature and possible consequences of the study had been fully explained. The study protocol was reviewed and approved by the Ethics Committee of the National Center for Global Health and Medicine, Tokyo.
Variables and measurements
Anthropometric and blood measurements
WC, VFA and SFA were measured by using a CT scanner, the details of which have been described elsewhere.25 In brief, single slice imaging was performed at the umbilical level in a supine position (Redix Turbo; Hitachi Medico, Chiyoda-ku, Tokyo). The imaging conditions were 120 kV, 50 mA, with a slice thickness of 5 mm. WC, SFA and VFA were calculated by using the PC software application fatPointer (Hitachi Medico). Body height and weight were measured by using an automated scale (BF-220; TANITA; Itabashi-ku, Tokyo) with the subjects wearing a light gown. BMI was calculated as body weight in kg divided by the square of body height in meter.
Sleep duration was self-reported. The average sleep duration on weekdays was defined by the response to the question (as translated into English): ‘On average, how many hours do you sleep per day?’ The response categories included: ‘<5 h’, ‘5 to <6 h’, ‘6 to <7 h’ and ‘⩾7 h’.
Health-related lifestyles were ascertained by using a questionnaire. Participants entered their responses to the questionnaire directly into a computer using a custom-designed data entry system. Regarding the health conditions of participants , data were obtained from the routine health examination. Physical illness was defined as having been diagnosed with and/or being currently under treatment of at least one of the following diseases: hypertension, diabetes mellitus, hyperlipidemia, hyperuricemia, anemia, gastric ulcer, duodenal ulcer, colon polyp, chronic hepatitis, fatty liver, gallstone, disk hernia, rheumatoid arthritis, epilepsia, thyroid-gland-related diseases, angina, cardiac dysrythmia, tuberculosis, bronchial asthma, kidney diseases and other diseases. Similarly, psychiatric illness was defined as having been diagnosed with and/or being currently under treatment of any psychiatric diseases. Regarding cigarette smoking, the questionnaire inquired whether the participants were nonsmokers, ex-smokers or current smokers. Nonsmokers and ex-smokers were later combined for statistical analyses. For alcohol consumption, participants were asked whether they were nondrinkers or current drinkers. For current drinkers, the frequency of drinking and the amount of alcohol consumed per session was assessed in terms of go (one go contains ∼23 g of ethanol). A yes/no question was used to assess regular physical activity.
To explore gender differences, all data analyses were conducted separately in men and women. Characteristics of participants are presented as numbers (percentages) for categorical variables and mean with s.d. for continuous variables. Statistical differences in characteristics and anthropometric measurements and according to sleep duration categories in men and women were assessed using χ2 test or Fisher's exact test for categorical variables and t-test or one-way analysis of variance for continuous variables.
Analysis of covariance was used to estimate adjusted means of BMI, WC, VFA and SFA across categories of sleep duration (<5 h, 5 to <6 h, 6 to <7 h and ⩾7 h). In model 1, we adjusted for age (continuous), regular physical activity (yes or no), current smoking status (nonsmokers or smoker) and current alcohol drinking (nondrinkers or drinker). Because physical illness has been found to be a potential confounder in the association between sleep duration and obesity,21 we included it in model 2 in addition to the covariates in model 1. An additional model (model 3) was constructed for VFA and SFA. In addition to the covariates in model 2, SFA was included in the model for VFA, and VFA was included in the model for SFA. Trend of the association was assessed by using multiple linear regression models with ordinal numbers of 0 to 3 assigned to the categories of sleep duration with adjustments for the same covariates included in each model of analysis of covariance. In addition, statistical tests for a gender interaction were performed by including pair-wise interaction terms (that is, BMI × gender, WC × gender, VFA × gender and SFA × gender) in the multiple regression models. Two-sided P-values of <0.05 were regarded as statistically significant. We used IBM SPSS Statistics version 19.0 (IBM Corporation, New York, NY, USA) for all the statistical analyses.
The study subjects included 5400 men and 642 women with an age range between 30 to 75 years (mean=53.3 years±10.0 years in men and mean=58.2 years ±9.4 years in women). Regarding the average sleep duration, 5.4% of the total study population slept <5 h, 42.6% slept 5 to <6 h, 39.4% slept 6 to <7 h and 12.6% slept ⩾7 h per day. The proportion of subjects sleeping >6 h per day was significantly higher in men than in women (53.0% vs 47.5%). Men were also significantly more likely to be current cigarette smokers (33.9% vs 3.3%) and current alcohol drinkers (76.4% vs 21.2%). The mean value of BMI, WC and VFA was significantly higher in men (mean=24.2±3.1 kg m–2, mean=86.7±8.3 cm and mean=124.2±53.8 cm2, respectively) than in women (mean=23.1±3.3 kg m–2; mean=83.6±9.5 cm; and mean=82.8±45.1 cm2; respectively). However, the mean value of SFA was significantly lower in men (mean=136.5±57.8 cm2) than in women (mean=185.9±75.8 cm2). Men were significantly less likely to be living with at least one physical illness compared with women (37.1% vs 47.8%).
Tables 1 and 2 show characteristics of subjects according to sleep duration categories in men and women, respectively. In men, sleep duration increased as age increased. Men with shorter sleep duration were more likely to be current cigarette smokers and to be living with at least one physical illness. However, men with shorter sleep duration were less likely to be current alcohol drinkers, and they were less likely to involve with regular physical activity compared with those with longer sleep duration. In men, mean values of BMI, WC and SFA significantly decreased as sleep duration increased. In women, no significant association was found between characteristics of subjects and sleeping duration.
Table 3 shows the adjusted mean values of anthropometric indexes of subjects according to sleep duration categories in men. After adjustment for age, regular physical activity, cigarette smoking and alcohol drinking in model 1, mean values of BMI, WC and SFA decreased significantly with increasing sleep duration (P-values for trend <0.001). Adjustment for physical illnesses (model 2) did not significantly change the explanatory power of the models. For subjects sleeping ‘<5 h’, ‘5 to <6 h’, ‘6 to <7 h’ and ‘⩾7 h’ per day, mean values of BMI were 24.8±3.5 kg m–2, 24.3±3.2 kg m–2, 24.0±2.7 kg m–2 and 23.8±2.6 kg m–2, respectively (P-values for trend <0.001), and mean values of WC were 87.9±9.3 cm, 86.9±8.7 cm, 86.4±7.8 cm and 85.7±7.5 cm, respectively (P-values for trend <0.001). The significant inverse association between sleep duration and SFA was also not attenuated after additional adjustment for physical illnesses (model 2) and VFA (model 3). In fully adjusted model, the mean values of SFA for subjects sleeping ‘<5 h’, ‘5 to <6 h’, ‘6 to <7 h’ and ‘⩾7 h’ per day were 145.8±67.4 cm2, 138.7±61.5 cm2, 134.7±60.4 cm2 and 132.5±49.2 cm2, respectively (P-values for trend <0.001). Sleep duration was not appreciably associated with VFA in men. As shown in Table 4, sleep duration was not significantly associated with BMI, WC, VFA or SFA in any models in women. Gender interaction tests were all statistically significant for all the outcomes of interest (all P-values <0.001).
In this cross-sectional study, we investigated the relationship of sleep duration with general obesity and abdominal fat areas. To the best of our knowledge, this is the first study of its kind in which CT scanner was used to measure WC, VFA and SFA. We found that short sleep duration was strongly associated with higher BMI, WC and SFA in men. The association was independent of the effects of potential confounding factors such as physical and psychiatric illnesses. However, sleep duration was not appreciably associated with VFA. Apparent gender differences were observed as significant relationship was not detected between sleep duration and any obesity-related measures in women.
This study extends the understanding in the literature of sleep and obesity research in which sleep deprivation has been considered as a potential predictor of obesity, frequently defined by using BMI. BMI is not a valid proxy for body fat mass, and existing BMI cutoffs are not suitable for the classification of individuals as normal weight, overweight or obese in Asians.26 Asians have proportionally more fat for a similar BMI level and are at increased cardiovascular risk at lower BMI levels as compared with Caucasians.27 Findings from our study suggest that, in working Japanese men, short sleep duration is associated not only with general obesity, but also with increased subcutaneous fat mass, supporting a role of chronic sleep restriction in obesity pathogenesis.
Reviews of several cross-sectional and prospective studies among child and adult populations around the world have found fairly uniform results that short sleep duration is associated with obesity7 and weight gain.16 However, in their most recent review of prospective studies, Nielsen et al.2 concluded that short sleep duration is consistently associated with development of obesity in children and young adults, but the findings were less consistent in older adults. Furthermore, sleep duration was not associated with BMI in a population-based cohort study among Japanese aged 40 to 69 years,17 as well as in a prospective multicenter cohort study among early-middle-aged adults (age range of 38 to 50 years) in the United States.18 In a German study, the significant association between short sleep duration and BMI did not persist after controlling for physical health and emotional status.21
This study is the first in the field to formally assess physical and psychiatric illnesses and to assess the relationship between sleep duration and obesity independent of these factors. These confounders may lead to a relationship in the opposite direction; obesity predisposes to physical or psychiatric illnesses, which in turn cause reduced sleep duration. Previous studies have ignored this explanatory pathway or attempted to address it by using self-reported data obtained from a single question on the overall physical and mental health of the participants.21 Such a measure is not sensitive and does not capture severity of the illnesses. In our analyses, we were not able to show a significant attenuation of the association between short sleep duration and obesity after excluding subjects with psychiatric illnesses and controlling for physical illnesses. These findings suggest that the association among our study population may not be explained by this pathway. Further studies are needed to investigate the possible confounding effects of physical and mental disorders on the relationship between sleep duration and obesity.
It is worth noting that short sleep duration did not show any significant association with general obesity and central abdominal fat areas among women in this study. Similar findings were also found in a study among a large Japanese working population in which no prospective association between sleep duration and obesity or weight gain was detected in women.19 This finding is also consistent with results obtained from other studies in western populations.28 In contrast, a study in Spain showed that the significant association between sleep duration and weight gain was observed in women, but not in men.20 However, direct comparison with men might be made with caution as the mean age of women in our study was roughly 5 years older than that in men. In the Zurich Cohort Study, the relationship between sleep duration and weight weakened as participants aged.29 Furthermore, our bivariate results show that sleep duration was not related to any obesity-related characteristics in women.
Based on experimental studies of sleep deprivation, a number of causal pathways linking short sleep duration with obesity have been suggested. One mechanism by which sleep deprivation might predispose to weight gain is by increasing caloric intake. In short-term trials, sleep restriction leads to reduction in circulating leptin, elevations in ghrelin, subjective hunger and preferences for calorie-dense, refined-carbohydrate foods,30 which contribute to the development of obesity. Alternatively, some have argued that, in an environment where food is readily available, curtailed sleep may simply represent an increased opportunity to eat, especially if most of the wake-time is spent in sedentary activities such as watching television where snacking is common.31 Chronic sleep deprivation clearly leads to feeling fatigue that may in turn lead to obesity-related behavior including decreased energy expenditure, irregular eating habit and low consumption of fruits and vegetables.32 In addition, activation of inflammatory pathways by sleep restriction may also be implicated in the development of obesity.33
The strengths of this study include the large sample size of men, the use of CT scanner to measure central abdominal fat areas and the comprehensive assessments of important covariates. The relationship between sleep duration and obesity may vary in association with underlying risk factors such as insomnia and psychological disorders that are potential comorbidities of sleep deprivation and other severe medical conditions that might affect body composition. With a broad variety of data obtained from a standardized collection, we were able to exclude subjects with a history of psychiatric illnesses, insomnia, stroke, myocardial infarction, cancer and diabetes mellitus. In this way, we extended previous findings by systematically assessing the association between sleep duration and obesity independent from the effects of these potential confounding factors.
Several limitations should also be recognized. First, because of the cross-sectional design, a causal relationship cannot be definitively established. However, experimental studies have confirmed that sleep restriction can have metabolic effects that may be relevant to weight homeostasis.31 Future studies should evaluate how changes in sleep duration are related to changes in weight and body fat composition over time. Second, daily sleep duration was self-reported, which is a continued limitation in sleep epidemiological studies. However, the Nurses’ Health Study has shown a good validity for sleep duration measured by using a similar question against 1-week sleep diaries.34 Third, long sleepers (>8 h) were not specifically separated from normal sleepers (7 to 8 h). As a result, we were unable to examine the relation between long sleep and obesity, as many studies have reported a U-shaped association.12, 35 Furthermore, information regarding sleep duration did not allow us to distinguish the real ‘sleep duration’ and ‘time in bed’. Finally, although we excluded subjects with history of insomnia, no adjustment was made for other important sleep disorders such as obstructive sleep apnea, which is presumed to play an important role in both sleep disruption and obesity.36 Future research should examine whether obstructive sleep apnea accounts for the gender differences in the association between sleep duration and adiposity as previous studies found that Asian men appear to have an increased risk of obstructive sleep apnea at lower BMI levels than observed in Caucasian men.37
In conclusion, our findings suggest that short sleep duration is associated not only with general obesity, but also with subcutaneous fat mass in Japanese working men. Further research is needed to further explicate the biological mechanisms behind this relationship and to see whether interventions addressing inadequate sleep or poor sleep quality could treat or prevent obesity by taking gender differences into consideration.
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Financial support for this study was provided by a grant from the Ministry of Health, Labor, and Welfare of Japan. We are grateful to the study participants for their contribution.
The authors declare no conflict of interest.
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Cite this article
Yi, S., Nakagawa, T., Yamamoto, S. et al. Short sleep duration in association with CT-scanned abdominal fat areas: the Hitachi Health Study. Int J Obes 37, 129–134 (2013). https://doi.org/10.1038/ijo.2012.17
- sleep duration
- general obesity
- central obesity
- abdominal fat area
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