OBJECTIVE: To observe the effect of transcutaneous auricular vagus nerve stimulation (taVNS) on the sleep quality and nocturnal heart rate variability (HRV) in patients with primary insomnia. METHODS: Twenty-one patients with primary insomnia were included. Using SDZ-鈪 electric acupuncture apparatus, Xin (CO(15)) and Shen (CO(10)) were stimulated with disperse-dense wave, 4 Hz/ 20 Hz in frequency, (0.2+/-30%) ms of pulse width and tolerable intensity. Electric stimulation was given once every morning and evening of a day, 30 min each time, for 4 weeks totally. Before and after treatment, the score of Pittsburgh sleep quality index (PSQI), objective sleep structure (total sleep time [TST], sleep latency [SL], wake after sleep onset [WASO], sleep efficiency [SE], the percentages of non-rapid eye movement period 1, 2, 3, and the percentage of rapid eye movement period to TST [N1%, N2%, N3%, REM%] ) and nocturnal HRV (high frequency [HF], low frequency [LF], the ratio of LF to HF [LF/HF], standard deviation for the normal RR intervals [SDNN], squared root of the mean sum of squares of differences between adjacent intervals RR [RMSSD], the percentage of adjacent RR intervals with differences larger than 50 ms in the entire recording [PNN50%], the mean of sinus RR intervals [NNMean] ) were compared in the patients separately. RESULTS: After treatment, the score of each item and the total score of PSQI and SL were all reduced as compared with those before treatment (P<0.01, P<0.001); SE, N3%, LF, HF, LF/HF, SDNN, NNMean and RMSSD were all increased compared with those before treatment (P<0.001, P<0.01). CONCLUSION: The taVNS improves the sleep quality and objective sleep structure in patients with primary insomnia, which is probably related to the regulation of autonomic nervous functions.