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Background Obstructive sleep apnea syndrome (OSAS) is strongly associated with obesity and with cardiovascular disease.Ghrelin and obestatin are two peptides from the same source but have opposite roles.Both of them can affect feeding and regulate vascular tune.The aim of this study was to investigate the relationship between plasma ghrelin,obestatin,the ratio of ghrelin and obestatin (G/O) and sleep parameters and blood pressure circadian rhythms in patients with OSAS.Methods This study enrolled 95 newly diagnosed over-weight OSAS patients (OSAS group),30 body mass index (BMI)-match non-OSAS adults (over-weight group) and 30 non-OSAS normal weight adults (control group).Polysomnography (PSG) was performed in the OSAS group and over-weight group.Blood pressure of all subjects was monitored by means of 24-hour ambulatory blood pressure monitoring.The concentration of plasma ghrelin and obestatin was detected by enzyme-linked immunosorbent assay (ELISA).Results Plasma ghrelin levels in the OSAS group and over-weight group were significantly lower than that of the control group (P ＜0.05).Plasma obestatin levels were lower in the over-weight group and OSAS group,but there was no significant difference among the three groups.The blood pressure in OSAS patients was higher,and there was a significant difference in all blood pressure parameters compared to the control group,and in the daytime average diastolic blood pressure (DBP),noctal average systolic blood pressure (SBP) and DBP,DBP variability values as compared to over-weight subjects.Furthermore,there were significantly more non-dipper patts of blood pressure (including hypertension and normotension) in the OSAS group than in the other two groups (P ＜0.01).Correlation analysis showed that ghrelin levels had a significant correlation with BMI and noctal average DBP but not with PSG parameters.In contrast,the G/O ratio had a negative correlation with apnea-hypopnea index (AHI) (P ＜0.05),as well as a strong positive correlation with the blood pressure variability values (P ＜0.01).In multivariate analyses,AHI (P ＜0.05) and G/O (P ＜0.05)were independently related to SBP variability changes,while AHI (P ＜0.05),G/O (P ＜0.01) and BMI (P ＜0.05) were independently related to DBP variability changes.Conclusions Our data show plasma ghrelin and obestatin levels were related to obesity in OSAS.Sleep apnea in OSAS patients could have led to an imbalance in G/O in the basis of obesity.Moreover,the imbalance may promote nighttime blood pressure elevation and affect blood pressure circadian disorder.