BMC Public Health 2009, 9:130 http://www.biomedcentral.com/1471-2458/9/130 hours of daily sleep were less likely to report good sleep quality than those reporting more than 9 hours of daily sleep. Presence of CVD also affected the levels of sleep quality among the total population but when we stratified by gender and residential location this effect remained sig- nificant only among Beijing men and Shanghai women. We did not find any significant association between sleep quality and different lifestyle characteristics (smoking and alcohol drinking habits), age and BMI. This is consistent with Elwood et al. [38] who studied 1986 welsh men aged 55–69 y and found no significant association between prevalence of insomnia and age, BMI, smoking and alco- hol drinking habits. However, it is possible that the effect of smoking and alcohol drinking in our analysis might be confounded with gender as the majority of Chinese smok- ers and drinkers were men. Furthermore, this is affected by the limited number of participants in the extreme catego- ries of alcohol consumption and BMI, which limits the statistical power to identify a potential association between these two variables and sleep patterns in our population. Despite lower socio-economic status (lower average income, fewer years in full-time education and less medi- cal insurance coverage) rural residents were more likely to report good levels of sleep quality compare to urban resi- dents. This could be due to differences in lifestyle wherein those who live in rural areas tend to have higher levels of physical activity, higher age of retirement, less stressful lifestyles and a lower prevalence of both CVD (7.7% rural vs. 13% urban) and diabetes (11.4% rural vs. 17% urban). In our analyses of the total sample and subgroups of the sample, we found no significant effects of income and education. This could be due to the fact that residence is a more comprehensive factor that might partially confound the effect of socio-economic factors (i.e. our sample of the Chinese population is skewed towards lower income and education and late retirement in rural areas). Concentrations of inflammatory biomarkers, plasma insulin and index of insulin resistance were associated with sleep quality for the total population and among a subgroup of participants that were free from CVD and dia- betes. However, no clear trends in the levels of biomarkers as a function of the different levels of sleep quality (good/ common/poor) were observed. This is the first study to evaluate the associations between sleep quality and inflammatory markers and measures of insulin resistance in China and to evaluate the distribution and factors asso- ciated with sleep quality in such a vast country which has many geographical and residential differences Although we considered the effect of comprehensive measurements of lifestyle, socio-demographic factors and presence of disease, our findings were limited. This could be due to the cross-sectional nature of our data and fur- ther longitudinal studies should be carried out to confirm our findings. Furthermore, sleep patterns were measured using a ques- tionnaire; a criticism of this type of measurement is that answers are provided by the participants and can be affected by recall bias. Furthermore, due to the size of the population and the extent of the general questionnaire, for logistic reasons the questions that we used are not as thorough as alternative validated questionnaires designed to evaluate sleep disturbances (e.g The Pittsburg Sleep Quality Inventory). Nevertheless, these factors could only generate a non-differential misclassification of the expo- sure (sleep patterns) that would attenuate the size of the effects hereby reported and the level of sleep disturbances in the population studied, but not necessarily our results and conclusions. Prospective evaluations and studies that provide more sensitive measures of sleep patterns in Chi- nese populations are required to clarify the determinants that affect sleep quality. This will also allow us to assess whether sleep disturbances are associated with early devi- ations from adequate cardio-metabolic health. Conclusion In general, only half of the population reported good sleep quality. Compared to their counterparts, men and people living in south China were twice as likely to report good sleep quality. Both physical and mental health sig- nificantly affected the quality of sleep in elderly Chinese and having enough daily sleep hours substantially increased the likelihood of reporting better sleep quality. Although we did not find any clear trends between levels of sleep quality and levels of inflammatory markers, we did find a significant association between levels of sleep quality and resistin, insulin, HOMA-IR, HOMA2-IR, LDL, and TCH. These associations were found among the total population and a subgroup of participants free from CVD and diabetes. Considering the alarming increase of sleep disturbances that modern societies are experiencing and the deleterious consequences that a poor night sleep can exert on our metabolism and health, it is fundamental to clarify the factors that might generate sleep disturbances, how these disturbances can affect the biological homeostasis and identify potential interventions to prevent sleep distur- bances at an early stage. Abbreviations ADIPO: adiponectin; ANOVA: analysis of variance; BMI: body mass index; CI: confidence interval; CRP: C-reactive Page 9 of 11 (page number not for citation purposes)