Objective:To evaluate the efficacy and safety of modified suanzaorentang,on primary insomnia over a period of 4 weeks on both subjective and objective sleep quality.Methods:A prospective,randomized,double-blind,placebo-controlled trial (RCT) involving 162 primary insomniac subjects (mean age 47 years,range 21 to 64 years;female:68.5％) was conducted at 2 university hospital sleep centers in Hong Kong.Among 162 subjects,86％ (n =139) completed the whole trial.Participants received either 4 weeks Chinese herbal formula (modified suanzaorentang group) or placebo by block randomization.Main outcome measures were subjective measures of sleep quality,and objective outcome measures including sleep onset latency,wake after sleep onset,sleep efficiency and sleep stages derived from polysomnography,and sleep variability from actigraphy.Results:After 4-week of treatment,the treatment group showed more improvement in subjective sleep quality as measured 100 mm visual analogue scales [deepness of sleep,95％ CI =-16.0(-22.1--9.9) vs.-7.1 (-13.3--1.0),P ＜ 0.05;refreshing sleep,95％ CI =-12.0(-18.2--5.8) vs.-2.2(-8.9-4.5),P ＜ 0.05] than placebo group.However,the two groups did not have any differences over the objective measures and the Insomnia Severity Index total score.No difference in overall adverse events was found between the two groups.Conclusion:The study shows that a short term (4 weeks) treatment of modified suanzaorentang could improve perceived sleep quality but not subjective sleep difficulty or objective sleep parameters.In particular,modified suanzaorentang has good safety profile and tolerability.