Background: Acupuncture is a potentially effective non-pharmacological treatment for insomnia. Objective: We observed the responses of patients with insomnia to acupuncture in routine clinical practice. In addition, we explored patient characteristics that might affect the treatment response to acupuncture for insomnia. Methods: Medical records of patients with insomnia in a Korean medicine clinic with baseline Insomnia Severity Index (ISI) scores ⩾8 and Pittsburgh Sleep Quality Index (PSQI) scores ⩾5 were reviewed. Acupuncture was applied at ST43, GB41, ST41, SI5, HT3, KI10, HT7 and ST3, for 1–2 months. The ISI and PSQI were measured monthly to assess insomnia severity. The effect of acupuncture over time was analyzed using a multilevel linear model for repeated measures. In addition, logistic regression was used to explore predictors of treatment response. Results: A total of 91 patients with insomnia aged 59.2 ± 12.5 years (mean ± standard deviation (SD)) (90.1% female) were included in the analysis. After the acupuncture treatment, ISI scores were significantly reduced by −3.75 (95% confidence interval (CI) = −4.99, −2.50) and −4.69 (95% CI = −6.22, −3.16) after the first and second month, respectively. The PSQI global scores also improved, and sleep duration showed a tendency to increase by 0.35 h (95% CI = −0.17, 0.86) after acupuncture treatment. Three cases of mild fatigue were reported. In addition, higher baseline pain/discomfort predicted a greater likelihood of response after acupuncture treatment (odds ratio (OR) = 1.66, 95% CI = 1.10, 2.60). Conclusion: In a real-world setting, the insomnia of outpatients in a clinic was slightly alleviated after acupuncture treatment. These findings require validation by randomized controlled trials.