Purpose <p>Chronic insomnia is common. It causes impaired daytime function. Non-pharmacological options are needed. Transcranial alternating current stimulation is a non-invasive technique. It delivers weak electrical currents to the scalp. It aims to modulate abnormal brain activity.</p> Methods <p>We performed a systematic review. We also conducted a meta-analysis. Randomized controlled trials were included. Studies compared tACS with sham stimulation. Major databases were searched. Searches were conducted from inception to September 2025. Sleep quality was the primary outcome. It was assessed using the Pittsburgh Sleep Quality Index. Sleep onset latency was a secondary outcome. Total sleep time was also evaluated. Risk of bias was assessed using the Cochrane ROB-2 tool. A random-effects model was applied. Five trials met inclusion criteria.</p> Results <p>A total of 334 participants were analyzed. tACS significantly improved sleep quality. PSQI scores were reduced. Sleep onset latency was shorter. Total sleep time increased. Moderate to high heterogeneity was observed. Sensitivity analyses supported the results. tACS was associated with improved sleep outcomes. Benefits were observed across multiple domains. tACS may represent a useful non-pharmacological treatment option. Further trials are required. Longer follow-up is needed.</p> Conclusion <p>Transcranial alternating current stimulation (tACS) appears to improve sleep quality in adults with chronic insomnia, reducing sleep onset latency and increasing total sleep time compared with sham stimulation. However, the evidence is based on a small number of short-duration trials with heterogeneity, and larger well-standardized RCTs with longer follow-up and objective sleep measures are needed.</p>