Objective To explore serum level of 25-hydroxy vitamin D3 (25 [OH]D3) and its correlations with sleep quality and cognitive function in patients with chronic insomnia disorder (CID).Methods Fifty CID patients,admitted to our hospital from October 1,2016 to September 1,2018,and 40 healthy controls were enrolled.Pittsburgh Sleep Quality Index (PSQI) was used to assess insomnia severity (objective sleep status being assessed using polysomnography throughout a night in some patients).Montreal Cognitive Assessment scale (MoCA) was used to evaluate the general cognitive function.Serum level of25(OH)D3 was detected by ELISA.Results As compared with the controls,CID patients had significantly higher PSQI scores (P＜0.05) and statistically higher Hamilton's Depression Scale-17 scores (P＜0.05).The CID patients had significantly lower MoCA scores,having more errors in object work,spatial work and object recognition memorizes in Nine-Box Maze Test than the controls.Partial correlation analysis showed that in CID patients,the level of 25(OH)D3 was negatively correlated with PSQI scores (r=-0.320,P=0.030),spatial working memory errors (r=-0.300,P=0.043) and disease course (r=-0.360,P=0.014),whereas it was positively correlated with total sleep time (r=0.515,P=0.014)and MoCA scores (r=0.422,P=0.003).Conclusion The level of 25 (OH)D3 decreases in the CID patients,which is correlated with insomnia severity and course,and may affect the cognitive function.