BMC Public Health 2009, 9:130 http://www.biomedcentral.com/1471-2458/9/130 Table 5: Cardio-metabolic factors and biomarkers by levels of sleep quality Variable, mean (SD) Resistin PAI-1 RBP4 Adiponectin IL-6 CRP Insulin HOMA-IR HOMA2-IR SBP DBP HDL LDL TCH TG Total 11.47(9.37) 14.74(18.17) 40.11(11.74) 16.48(11.62) 1.55(2.78) 1.58(4.30) 15.46(9.50) 4.10(3.36) 0.31(0.22) 140.11(22.49) 80.16(10.79) 1.28(0.33) 3.26(0.97) 4.70(0.98) 1.39(1.07) well 11.26 (8.92) 14.35 (18.71) 39.78 (11.59) 16.47 (11.58) 1.46 (2.13) 1.42 (2.29) 15.00 (9.37) 3.95 (3.36) 0.29 (0.19) 139.5 (22.06) 80.18 (10.74) 1.27 (0.33) 3.19 (0.94) 4.63 (0.97) 1.36 (1.07) common 11.24 (8.92) 15.44 (17.56) 40.48 (11.58) 16.06 (11.41) 1.68 (3.63) 1.72 (5.54) 16.07 (8.86) 4.32 (3.25) 0.32 (0.27) 140.7 (22.94) 80.15 (10.82) 1.27 (0.33) 3.36 (0.99) 4.79 (0.97) 1.44 (1.12) poor 12.57 (11.34) 14.71 (17.50) 40.47 (12.48) 17.29 (12.11) 1.61 (2.84) 1.82 (6.24) 15.81 (10.93) 4.14 (3.55) 0.31 (0.22) 141.1 (22.99) 80.11 (10.92) 1.30 (0.36) 3.30 (1.01) 4.75 (1.02) 1.39 (0.96) p-value 0.0428* 0.0011* 0.2924 0.1914 0.4680 0.9505 <0.0001* <0.0001* <0.0001* 0.2338 0.9892 0.5415 <0.0001* 0.0001* 0.0490* Abbreviations: PAI-1, plasminogen activator inhibitor-1; RBP4, retinol binding protein 4; IL-6, interleukin-6; IL6, interleukin-6; CRP, C-reactive protein; HOMA-IR, homeostasis model assessment of insulin resistance (fasting glucose (mmol/L) × fasting insulin (μU/ml)/22.5); SBP, systolic blood pressure; DBP, diastolic blood pressure; HDL, high-density lipoprotein; LDL, low-density lipoprotein; TCH, total cholesterol; TG, triglycerides; SD, standard deviation; of sleep quality. We also observed significant gender dif- ferences in those who reported poor sleep quality wherein Chinese women were almost 50% less likely to report good sleep quality than men. Similar to our findings, Li et al.[6] found a 12% prevalence of insomnia among Hong Kong Chinese adults. Li et al.[6] studied 9851 Hong Kong Chinese aged 18–65 years and found a 60% greater chance of insomnia being reported among women than men. Klink et al.[5] also found there was a 50% higher chance of women reporting insomnia. Furthermore, in a study of 400 adults aged 20–70 years, Reyner et al.[7] found women, reported more awakenings, more total time spent awake during the night and poorer sleep qual- ity. This difference in sleep quality could be due to gender differences in the prevalence of psychiatric morbidities, socio-cultural factors and coping strategies. [5-7] We also found that sleep quality related to geographical location. Those living in the north were 40% less likely to report good sleep quality compared to those living in the south. This could be due to differences between the two cities wherein compared to Shanghai residents, Beijing residents show almost twice the prevalence of chronic dis- eases (depression, CVD, diabetes and metabolic syn- drome), higher levels of alcohol consumption (37% Beijing vs. 20% Shanghai) and smoking (31% Beijing vs. 25% Shanghai), higher prevalence of nap taking (55% Beijing vs. 29% Shanghai) and less medical insurance cov- erage (53% Beijing vs. 86% Shanghai). To our knowledge this is the first study investigating differences in sleep quality as a function of geographical and residential status in China. The effect of SRH (which is considered to be a valid repre- sentative of physical and mental health[38-40]) was sub- stantial on sleep quality for the total population and for gender and geographical subgroups. Those who reported good/excellent SRH had twice the chance of reporting good sleep quality compared to those that had fair, poor or very poor SRH. In our study, we also found a strong association between sleep quality and presence of depressive symptoms, with the strongest effect among Shanghai men and Beijing women. The likelihood of reporting poor sleep quality was three times higher in the presence of depressive symp- toms. Our findings are consistent with other studies that have shown significant associations between depression and insomnia[41] and depression and sleep distur- bances.[42] The association between poor sleep and depression has also been observed in an elderly popula- tion.[2] Currently, the number of hours required for optimal func- tioning in an elderly population is not known. Although it is commonly believed that 8 hours of sleep per night is optimal for good health, recent studies suggest that mor- tality risk is lower among those sleeping 7 hours.[21] The total hours of daily sleep includes night time sleep and nap time (nap taking is common mainly in north of China) and for that reason, 7–9 hours was categorized as normal daily sleep hours in our study. We found a sub- stantial effect of sleep quantity on quality of sleep. Having a short sleep time significantly decreased the chance of reporting good quality of sleep. Even those reporting 7–9 Page 8 of 11 (page number not for citation purposes)