BMC Public Health 2009, 9:130 http://www.biomedcentral.com/1471-2458/9/130 Background Sleep disturbances, including chronic insomnia (difficulty in maintaining sleep, having intermittent awakening and early awakening) and sleep apnea are increasingly com- mon in modern societies with reported prevalences vary- ing between 4 and 22 percent in different populations.[1] Sleep disturbances are common in later life, with loss of nocturnal sleep continuity and depth. [2-4] This could be due to the high prevalence of diseases accompanied by pain and stress, the side effects of taking medications and the inherent changes in sleep architecture that tend to occur with age. Aside from age and gender, physical and mental health problems are reported to be significantly associated with sleep disturbances. [5-8] Across different populations, several studies have found a significant dele- terious effect of sleep disturbances on self-rated health, incidence of cardio-metabolic diseases (coronary heart disease, metabolic syndrome and type 2 diabetes), quality of life and mortality. [9-25] Various mechanisms have been explored and suggested as the potential link between poor sleep patterns and negative health outcomes (e.g. impaired glucose regulation and subsequent increased risk of type 2 diabetes[24]). Nevertheless the existing information is limited and the pathophysiological path- ways between poor sleep patterns and cardio-metabolic disease remain unclear. Furthermore, although ample data exist for different populations, there is limited infor- mation on the levels of sleep disturbances among Chinese populations, the possible factors associated and the potential association of sleep quantity and quality with biomarkers of early cardio-metabolic disorders. Therefore, in order to evaluate the levels of sleep patterns in Chinese and contribute to complement its related gap of epidemiological evidence and knowledge, in the cur- rent study we aimed to evaluate: (i) the distribution of sleep quality; (ii) factors that determine sleep quality; and (iii) whether self-reported sleep quality is associated with biomarkers of cardio-metabolic disease in middle-aged and elderly Chinese. Methods Study population We used data from "Nutrition and Health of Aging Popu- lation in China" study. [26-28] This is a cross-sectional survey conducted between March and June 2005, among 3289 subjects (1,458 men and 1,831 women) aged 50 to 70 years from urban and rural areas of Beijing and Shang- hai (metropolitan cities of north and south of China). The sampling was conducted with an emphasis on recruiting at least 40% of men, a similar number of rural/urban res- idents of Beijing and Shanghai and a representation of people from all levels of education and income. Trained physicians or public health workers completed the sam- pling through a face-to-face interview using a standard- ized questionnaire to collect socio-demographical, lifestyle and health related information. Further details about the questionnaire can be found elsewhere.[29] The study protocol was approved by the Institutional Review Board of the Institute for Nutritional Sciences. All participants provided written informed consents. Assessment of sleep and related factors Subjective sleep quality during the last month was recorded in 3 levels (well, common or poor). Sleep quan- tity was measured as self-reported average total hours of daily sleep during the last month, including both night and nap hours sleep and was categorized as under, normal and over (<7, 7 to 9 and >9 hours per day respectively). Socio-demographic variables included age (50–59 and 60–70 yrs), gender, geographic region (Beijing or Shang- hai), residence of living (urban or rural), educational attainment (low, moderate and high, based on the number of years of education as 0 to 6, 7 to 9, and ≥10 years respectively), total household income (low, moder- ate and high on the basis of less than 10,000, 10,000 to 29,999 and more than 30,000 Yuan annually), marital status (married or without spouse), employment status (grouped as currently employed, unemployed/on welfare system and retired) and social activity ('yes' or 'no' on the basis of participating in at least one of a list of social activ- ities such as painting, playing chess, singing and dancing). Lifestyle variables included smoking habit, defined as never, current and former (cessation of smoking >6 months); alcohol drinking (grouped into "yes "or "no") and physical activity level (low, moderate, or high) according to the IPAQ (International Physical Activity Questionnaire). Self rated health (SRH) was initially recorded in 5 levels (very good, good, fair, poor and very poor) and then com- bined into two categories: good (very good and good) and poor (fair, poor and very poor).[30,31] Pulmonary dis- ease was defined as existence of at least one of the follow- ing: asthma, chronic bronchitis, emphysema or chronic cor-pulmonale diseases. Presence of diabetes was defined as either measured fasting glucose greater than 7.0 mmol/ L or being diagnosed with type 2 diabetes or taking anti- diabetes drugs or insulin. Presence of cardiovascular dis- ease (CVD) was defined as one or more definite manifes- tations of coronary heart disease or stroke. The Center for Epidemiologic Studies Depression Scale (CES-D)[32], which has been previously validated in Chi- nese populations[33,34], was used to measure the pres- ence of clinically relevant depressive symptoms. Using the accepted cut-off point of 16 for the sum of scores, a CES- Page 2 of 11 (page number not for citation purposes)