ABSTRACT
We examined the relationship between sleep duration and arterial stiffness among a multi-ethnic cohort, and whether the associations differed among ethnic minority groups in the Netherlands. Data were derived from 10 994 participants (aged 18–71 years) of the Healthy Life in an Urban Setting (HELIUS) study. Self-reported sleep duration was categorized into: short (<7 h/night), healthy (7–8 h/night) and long (≥9 h/night). Arterial stiffness was assessed by duplicate pulse-wave velocity (PWV in m/s) measurements using the Arteriograph system. The association of sleep duration with PWV was analysed using linear regression (β) with 95% confidence interval (CI). Results showed that neither short nor long sleep was related to PWV in all ethnic groups, except for long sleep in Dutch men which was associated with higher PWV (indicating stiffer arteries) after adjustment for potential confounders (β = 0.67, 95%CI, 0.23–1.11). Our study showed no convincing evidence that sleep duration was related to arterial stiffness among various ethnic groups. The link between sleep duration and cardiovascular outcomes does not seem to operate through arterial stiffness. Further research is needed to consolidate these findings.
1. Introduction
Evidence shows that sleep is involved in many pathophysiologic events in the body, including changes in heart rate (Elsenbruch et al., Citation1999), regulation of immunity (Krueger & Majde, Citation2003), changes in blood clotting activity (Heiser et al., Citation1997; Irokoawa et al., Citation1998; Muller et al., Citation1989), mental health (Bokyo et al., Citation2013), cardio-metabolic diseases (Knutson, Citation2010) and related risk factors (Kahn et al., Citation2005). Several studies have shown an independent association between both short and long sleep duration and cardiovascular diseases (CVD) (Qureshi et al., Citation1997; Wingard & Berkman, Citation1983).
The underlying mechanism of the association between sleep duration and CVD is not clear, hence remains a subject of further investigation. Arterial stiffness predicts CVD events and all-cause mortality (Meguro et al., Citation2009; Mitchell et al., Citation2010; Vlachopoulos et al., Citation2010; Xiong et al., Citation2012). Arterial stiffness is regarded as an early marker of CVD, and considered as an important etiological pathway leading to cardiovascular complications (O’Rourke et al., Citation2002). Relationship between sleep duration and arterial stiffness may exist, as both share common determinants, for instance, both are associated with inflammation, hormonal changes and metabolic risk factors; both share common cardiovascular risk factors, and both are associated with increased risk of CVD. Arterial stiffness could also play a role in the link between sleep duration and cardiovascular outcomes. However, the few studies that assessed the relationship between sleep duration and arterial stiffness show some degree of inconsistency. For instance, one study found that long sleep, not short sleep duration, was associated with increased pulse-wave velocity (PWV in m/s) but only in men (Yoshioka et al., Citation2011). By contrast, Sunbul et al. (Citation2014) found that sleep deprivation was associated with increased PWV in healthy adults, whereas Wolff and colleagues found that both short and long sleep were associated with increased carotid intima-media thickness (IMT) (Wolff et al., Citation2008).
Relationship between sleep duration and PWV has not been investigated among ethnic minority groups, who are known to have shorter sleep durations and more CVD risk compared with their host European populations (Agyemang et al., Citation2005; Anujuo et al., Citation2014). Therefore, investigation of the relationship between arterial stiffness and sleep duration among various ethnic groups could help to gain insight into the role of sleep in CVD and on ethnic differences in cardiovascular outcomes. The purpose of the present study, therefore, was to investigate the association between sleep duration and arterial stiffness in a multi-ethnic cohort, and whether the associations differed among ethnic minority groups in the Netherlands.
2. Materials and methods
2.1. Study population
The current study was based on baseline data from the HELIUS (Healthy Life in an Urban Setting) study. The aims and design of the HELIUS study have been described elsewhere (Stronks et al., Citation2013). In brief, HELIUS is a large-scale cohort study on health and healthcare among different ethnic groups living in Amsterdam. The study started in 2011 and it includes individuals aged 18–70 years from the six major ethnic groups in Amsterdam (African-Surinamese, South-Asian Surinamese, Turkish, Moroccan, Ghanaian and Dutch origin), and focuses on three major disease categories: CVD, mental health and infectious diseases. Participants were randomly sampled from the municipal registers, stratified by ethnicity. The study protocols were approved by the AMC Ethical Review Board. The study protocols conform to international ethical standards in line with the policy of Chronobiology International (Portaluppi et al., Citation2010). All participants provided written informed consent.
For the current study, we used baseline data that were collected until June 2014. Data from both questionnaire and the physical examination were available in 13 316 participants. For current analyses, participants with a Javanese Surinamese origin (n = 137), other/unknown Surinamese origin (n = 141) or other/unknown origin (n = 26) were excluded because of the small sample sizes. In addition, individuals with no data on sleep duration (n = 207) and/or PWV (n = 1811) were also excluded from the analysis. This resulted in a dataset of 10 994 participants, including 1793 Dutch, 1815 African-Surinamese, 1828 South-Asian Surinamese, 1600 Ghanaians, 1968 Turkish and 1990 Moroccans.
2.2. Measurements
2.2.1. Sleep duration
Participants were asked to provide information on the average number of hours they usually sleep at night. Sleep duration was assessed using the item, “How many hours do you sleep on average per night?”. Short sleep was defined as having less than 7 h of sleep per night, healthy sleep as 7–8 h/night and long sleep as having 9 or more hours of sleep per night. This categorization has been widely used in previous studies (Wolff et al., Citation2008).
2.2.2. Pulse-wave velocity
Participants visited the research location in the morning after an overnight fast and were asked to refrain from smoking before the visit. Arterial stiffness measurements were performed in duplicate after 10 min of supine rest using the Arteriograph system (Tensiomed Kft., Budapest, Hungary), and the mean of these two PWV measurements was used for analyses. The details of the measurement have been described elsewhere (Snijder et al., Citation2015). For the calculation of PWV, the Arteriograph system assumes that the late systolic peak is generated by reflected waves from the bifurcation of the aorta. Wave travel distance, that is from the heart to the bifurcation and back, is estimated as the shortest distance between the suprasternal notch and pubic symphysis using a tape measure. PWV (m/s) can then be calculated as the distance travelled by the pressure wave and the time difference between the early and late systolic peak. Variability and reproducibility of PWV measurements appear better with the Arteriograph compared with the Complior and Sphychmocor systems (Baulmann et al., Citation2008). PWV measured by the Arteriograph system generates similar PWV values as obtained by MRI (Rezai et al., Citation2013).
2.2.3. Other measurements
Weight was measured in light clothing only on a SECA877 to the nearest 0.1 kg. Height was measured without shoes with a portable stadiometer (SECA 217) to the nearest 0.1 cm. Blood pressure (BP) was measured using a validated automated digital BP device (Microlife WatchBP Home, Microlife AG, Heerbrugg, Switzerland) on the left arm in a seated position after the participant had seated for at least 5 min. All measurements were performed in duplicate; the mean of the two measurements was used in the analyses.
Fasting blood samples were taken to determine the concentration of glucose by spectrophotometry, using hexokinase as primary enzyme (Roche Diagnostics, Tokyo, Japan). Total cholesterol, triglycerides and high-density lipoprotein (HDL) cholesterol were determined by colorimetric spectrophotometry (Roche Diagnostics). Low-density lipoprotein cholesterol (LDL) was calculated according to the Friedewald formula (Friedewald et al., Citation1972).
Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2). Participants were considered obese if the BMI was ≥30 kg/m2.
Hypertension was defined as systolic BP ≥ 140 mmHg, or diastolic BP ≥ 90 mmHg, or being on BP-lowering medication, or self-reported hypertension. Type-2 diabetes was defined as increased fasting glucose ≥7 mmol/L, or current use of glucose-lowering medication, or self-reported diabetes. Dyslipidaemia was defined as total cholesterol (TC) >6.22 mmol/L, or high-density lipoprotein cholesterol (HDL-C) <1.04 mmol/L, or low-density lipoprotein cholesterol (LDL-C) >4.14 mmol/L, or triglyceride (TG) >1.69 mmol/L (Eckel et al., Citation2010; Goldberg, 2015), or use of lipid-lowering medication.
Educational level was determined using participant’s highest level of education (either in the Netherlands or in the country of origin). Participants were categorized into those who have never been to school or had elementary schooling only (first category), those with lower vocational schooling or lower secondary schooling (second category), those with intermediate vocational schooling or intermediate/higher secondary education schooling (third category) and those with higher vocational schooling or university (fourth category). For the current analyses, the first two categories were combined because of small numbers.
Marital status included married/registered/partnership, living together, unmarried/never married, divorced/separated or widowed. Alcohol intake in the past 12 months (yes/no) and smoking status (yes/no/ex-smoker) were obtained by questionnaire.
Habitual physical activity was measured using the SQUASH questionnaire (Wendel-Vos et al., Citation2003). The SQUASH questions about multiple activities refer to a normal week in the past months. We categorized participants according to the Dutch guideline for physical activity by summing up the number of days per week for each moderate- and high-intensity activity lasting at least 30 min. A total of 5 days resulted in participants being categorized as achieving the Dutch norm for physical activity (Wendel-Vos et al., Citation2003).
2.2.4. Ethnicity
Participant’s ethnicity was defined according to the country of birth of the participant as well as that of his/her parents and self-report. Specifically, a participant is considered as of non-Dutch ethnic origin if he/she fulfils either of the following criteria: (1) he or she was born abroad and has at least one parent born abroad (first generation); or (2) he or she was born in the Netherlands but both his/her parents born abroad (second generation) (Stronks et al., Citation2009). Of the Surinamese immigrants in the Netherlands, approximately 80% are of either African or South-Asian origin. Both subgroups were classified according to self-reported ethnic origin. Participants were considered as of Dutch origin if the person and both parents were born in the Netherlands.
2.3. Data analysis
Baseline data were expressed as percentages, means or median with interquartile range and reported stratified for ethnicity. The association between sleep duration and arterial stiffness was analysed by linear regression adjustment for potential confounders. We used healthy sleep (i.e. 7–8 h/night) as a reference category in the regression model. Independent factors were selected in the regression model based on the factors known to influence arterial stiffness and sleep duration relationship (Tsai et al., Citation2014; Wolff et al., Citation2008; Yoshioka et al Citation2011). The influence of potential confounders was determined by the change in regression coefficient before and after inclusion in the regression models. We stratified the analysis by gender because of interaction between sleep duration and gender (p = 0.000). There was no significant interaction between sleep duration and antihypertensive agents in all the ethnic groups. All analyses were performed using STATA 11.0 (Stata Corp., College Station, TX). A p-value of <0.05 was considered as statistically significant.
3. Results
3.1. Characteristics of the study population
and show the characteristics of the study population by gender and ethnic group. In men, mean PWV ranges from 7.58 m/s in Moroccans to 8.22 m/s in South-Asian Surinamese. Among women, mean PWV ranges from 8.02 m/s in Moroccans to 9.32 m/s in South-Asian Surinamese. Both men and women of South-Asian Surinamese, African Surinamese and Ghanaian origin had lower mean sleep duration and higher prevalence of short sleep than Dutch, Turkish and Moroccans. Prevalence of long sleep was higher in Moroccan, Turkish and South-Asian Surinamese men and women, compared with Dutch and other ethnic groups. Moroccan and Turkish men and women were younger, more often had lower educational levels, consume less alcohol, less often achieved the Dutch norm for physical activity and had lower prevalence of hypertension than Dutch and other ethnic groups. Dutch and South-Asian Surinamese men and women had lower BMI and were less obese than other ethnic groups. While prevalence of diabetes was higher in South-Asian Surinamese, Ghanaians and Moroccan men and women, as well as in African Surinamese and Turkish women, compared with Dutch, the prevalence of dyslipidaemia were lower in African Surinamese, Ghanaians and Dutch men and women as well as in Moroccan women compared with other ethnic groups.
Table 1. Characteristics of study population by ethnicity.
Table 2. Characteristics of study population by ethnicity.
3.2. Association between sleep duration and arterial stiffness
and show the crude associations between sleep duration and arterial stiffness in men and women, respectively.
Figure 1. Association between sleep duration and pulse wave velocity among ethnic groups in Amsterdam (Men).
Figure 2. Association between sleep duration and pulse wave velocity among ethnic groups in Amsterdam (Women).
Among men, mean PWV was higher in long sleepers in Dutch, South-Asian Surinamese and Ghanaians than short sleepers. By contrast, mean PWV was marginally higher among short sleepers in South-Asian Surinamese, African Surinamese, Dutch, Turkish and Moroccans than among long sleepers. In a multivariable model adjusting for age, marital status, mean arterial pressure, hypertension, diabetes, obesity and dyslipidaemia, smoking, alcohol consumption and physical activity, there was no significant difference in PWV between short sleepers and healthy sleepers in all ethnic groups except in Moroccans where PWV was lower in short sleepers (β = ‒0.21 m/s; 95%CI, ‒0.40 to ‒0.02) (). There was no significant difference in PWV between long sleepers and healthy sleepers in all ethnic groups, except in Dutch where PWV was higher in long sleepers (β = 0.67; 95%CI, 0.23–1.11).
Table 3. Regression coefficient (β) for the relationship between short sleep or long sleep (referenced to healthy sleep) and PWV (m/s) by ethnicity and gender.
Among women, mean PWV was higher in short sleepers in South-Asian Surinamese, Ghanaian and African Surinamese than long sleepers, whereas the mean PWV was marginally lower in both short and long sleepers in Moroccans, Turks and Dutch. In a multivariable model, however, there was no significant association between sleep duration and PWV (arterial stiffness) in all ethnic groups.
4. Discussion
4.1. Key findings
We investigated the association between sleep duration and arterial stiffness in a multi-ethnic cohort, and whether the associations differed among ethnic minority groups in the Netherlands. Our study showed that both long and short sleep duration was not associated with arterial stiffness as measured with PWV, except that PWV was higher in long sleepers as compared with healthy sleepers in Dutch men, and that PWV was lower in short sleepers in Moroccan men.
4.2. Discussion of key findings
The result of our study demonstrated that long sleep was not associated with arterial stiffness in all ethnic groups except in Dutch men. The positive association between long sleep and arterial stiffness in Dutch is consistent with previous studies among Japanese and Taiwanese populations. The mechanism for the association between long sleep and arterial stiffness is not clearly established. Previous studies suggest that the mechanism may be related to alterations in immunity (Grandner & Drummond, Citation2007) and pro-inflammatory processes that result in dysregulation of collagen and elastin fibres of the vascular wall, leading to arterial stiffness (Zieman et al., Citation2005). Although these proposed mechanisms may be involved in the association between long sleep and arterial stiffness, it is not fully understood how they may explain the relationship observed in Dutch men. It could be that selection mechanism may play a role, in which case, people with health problems sleep longer because of the health problems they are exposed to. Although we adjusted the association between sleep duration and arterial stiffness for physical activity, the degree of activity and the periods of inactivity may be different across ethnic groups and not fully be accounted for. One recent review, for example, highlighted the positive changes in arterial properties resulting from higher-intensity physical activity (Sacre et al., Citation2014). Age has also been shown to influence arterial stiffness (McEniery et al., Citation2005). However, the difference persisted after adjusting for age. Furthermore, we found no significant interaction between sleep duration and PWV in the Dutch population (p = 0.371) suggesting that age is unlikely to explain the observed association. The association between long sleep and arterial stiffness in Dutch seems to be associated with differences in risk factors for CVD because the association is attenuated after correction for CVD risk factors. This suggests that differences in risk factors for CVD, at least in part, contribute to the association between long sleep and arterial stiffness.
For ethnic minority groups, we found no association. The reason for lack of association in ethnic minority groups is unclear. We have previously shown that the relationship between sleep duration and CVD risk factors such as obesity, diabetes, hypertension, dyslipidaemia also differed between ethnic groups in this population (Anujuo et al., Citation2015). In this study, the relationship is also ethnic-specific. It has been suggested that the relationship between sleep duration and arterial stiffness may be explained by conventional CVD risk factors (Sunbul et al., Citation2014; Wolff et al., Citation2008). However, we did not observe a significant association between ethnic groups after adjusting for conventional CVD risk factors. Low socioeconomic status (SES) has also been suggested to play a role in the association between sleep duration and arterial stiffness (Sunbul et al., Citation2014; Wolff et al., Citation2008). Similarly, adjustment for SES did not change the results (data not shown). On the other hand, it could be that other unmeasured factors such as genetics may be involved in the differential associations of sleep duration with arterial stiffness among ethnic groups.
Our finding partly contrasts those of Wolff et al. (Citation2008), who found short and long sleep duration to be associated with increased carotid IMT in a general population-based study of men and women conducted in Germany. The difference in the result of our study, compared with that of Wolff et al. may be due to differences in outcome measure, for instance Wolff et al. used IMT, whereas in our study, PWV velocity was used. Although IMT has concordance with PWV, IMT is another marker of vascular aging and focuses on different stages in the atherosclerotic process, thus measuring more advanced structural changes of the vascular wall, whereas PWV targets the dynamic property based on vascular function and structure. Alternatively, the observed difference in results of the two studies may be due to differences in the age of participants. Participants were younger and healthier in our study (18–71 years) than those in the previous study (45–81 years), as long sleep and arterial stiffness have been shown to be significantly associated with older age (Woodard et al., Citation2011; Yoshioka et al., Citation2011).
With respect to short sleep duration, our study demonstrated that in both men and women short sleep was not positively associated with arterial stiffness in different ethnic groups. Our finding is consistent with previous studies conducted in Japan and Taiwan which documented that short sleep was not associated with arterial stiffness (Tsai et al., Citation2014; Yoshioka et al., Citation2011). Previous studies indicate that ethnic minority groups experience short sleep duration and increased prevalence of CVD risk factors and increased arterial stiffness (Agyemang et al., Citation2005; Anujuo et al., Citation2014; Snijder et al., Citation2015). Therefore, it was expected that the association between short sleep and arterial stiffness might be a link through which short sleep may predispose to CVD in ethnic minority groups in addition to independent association that has been previously suggested (Wingard & Berkman, Citation1983). Contrary to our expectation, we found no association. The reason for lack of association between short sleep and arterial stiffness among ethnic groups is not fully understood. Further study is needed to understand the potential reasons behind the lack of association between short sleep and arterial stiffness among ethnic groups.
Another finding from our study indicates that neither short sleep nor long sleep was associated with arterial stiffness in women. Our finding agrees with other studies which show gender-specific association of sleep duration with arterial stiffness (Tsai et al., Citation2014; Yoshioka et al., Citation2011). The reason for the observed non-significant association in women is unclear. Though previous study indicated that increased hormonal and C-reactive protein activity in perimenopausal or postmenopausal women may confer protection for arterial stiffness (Woodard et al., Citation2011). Nonetheless, the lack of association between sleep and arterial stiffness in women are the same for men (except Dutch), suggesting that hormonal differences may not be the likely explanation. Further studies are needed to explore the role of hormonal activity, and how it may contribute to the relationship between sleep duration and arterial stiffness in women.
The strength of our study lies in a large sample size, and hence more reliable estimations. Also multiple ethnic groups residing in one city were investigated together in a similar manner. However, there are limitations such as use of self-reported data for sleep, which is subject to recall bias, hence participants may have overestimated or underestimated sleep duration. Also information on daytime sleeping, sleep quality and use of hypnotics were lacking. The subjects included in this study were relatively younger than those in the earlier studies (Tsai et al., Citation2014; Yoshioka et al., Citation2011). This might be associated with the different results in this study compared with the earlier ones. Furthermore, the subjects with long sleep hours were fewer than those with short sleep durations. However, the number of long sleepers in each ethnic group was adequate for the analysis in the regression models. In addition, we did not exclude participants with obstructive sleep apnoea, which has been shown to be associated with arterial stiffness, due to lack of data on sleep apnoea in our study (Phillips et al., Citation2013).
In conclusion, the result of our study demonstrates that both short sleep and long sleep duration were not associated with arterial stiffness in different ethnic groups, except in Dutch men in which long sleep duration was associated with increased arterial stiffness. By contrast, in Moroccan men short sleep was associated with decreased stiffness. The link between sleep duration and cardiovascular outcomes does not seem to operate through arterial stiffness. Further study is needed from other European countries to consolidate these findings.
Declaration of interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.
Acknowledgements
The HELIUS study is conducted by the Academic Medical Center of Amsterdam and the Public Health Service of Amsterdam. Both organizations provided core support for HELIUS. The HELIUS study is also funded by the Dutch Heart Foundation (2010T084), the Netherlands Organisation for Health Research and Development (ZonMw): 200500003 and the European Union (FP-7): 278901. We acknowledge the AMC Biobank for their support in biobank management and high-quality storage of collected samples. We are most grateful to the participants of the HELIUS study and the management team, research nurses, interviewers, research assistants and other staff who have taken part in gathering data for this study.
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