Background: Insomnia is a common problem in modern society. It is associated with reduced quality of life and impairments in physical and mental health. Listening to music is widely used as a sleep aid, but it remains unclear if it can actually improve insomnia in adults. This Cochrane Review is an update of a review published in 2015. Objectives: To assess the effects of listening to music on sleep in adults with insomnia and to assess the influence of specific variables that may moderate the effect. Search methods: For this update, we searched CENTRAL, MEDLINE, Embase, nine other databases and two trials registers up to December 2021. In addition, we handsearched reference lists of included studies, and contacted authors of published studies to identify additional studies eligible for inclusion, including any unpublished or ongoing trials. Selection criteria: Randomised controlled trials comparing the effects of listening to music with no treatment or treatment as usual (TAU) in adults complaining of sleep difficulties. Data collection and analysis: Two review authors independently screened records for eligibility, selected studies for inclusion, extracted data and assessed risk of bias of the included studies. We assessed the certainty of the evidence using GRADE. The primary outcomes were sleep quality, insomnia severity, sleep-onset latency, total sleep time, sleep interruption, sleep efficiency and adverse events. Data on the predefined outcome measures were included in meta-analyses when consistently reported by at least two studies that were homogeneous in terms of participants, interventions and outcomes. We undertook meta-analyses using random-effects models. Main results: We included 13 studies (eight studies new to this update) comprising 1007 participants. The studies examined the effect of listening to prerecorded music daily, for 25 to 60 minutes, for a period of three days to three months. The risk of bias within the studies varied, with all studies being at high risk of performance bias, because of limited possibilities to blind participants to the music intervention. Some studies were at high risk of detection bias or other bias. Four studies reported funding from national research councils, three studies reported financial support from university sources and one study reported a grant from a private foundation. Five studies did not report any financial support. At the end of the intervention, we found moderate-certainty evidence for improved sleep quality measured with the Pittsburgh Sleep Quality Index (PSQI) in themusic groups compared to no intervention or TAU (mean difference (MD) −2.79, 95% confidence interval (CI) −3.86 to −1.72; 10 studies, 708 participants). The PSQI scale ranges from 0 to 21 with higher scores indicating poorer sleep. The size of the effect indicates an increase in sleep quality of the size of about one standard deviation in favour of the intervention. We found no clear evidence of a difference in the effects of listening to music compared to no treatment or TAU on insomnia severity (MD −6.96, 95% CI −15.21 to 1.28; 2 studies, 63 participants; very low-certainty evidence). We found low-certainty evidence that, compared to no treatment or TAU, listening to music may reduce problems with sleep-onset latency (MD −0.60, 95% CI −0.83 to −0.37; 3 studies, 197 participants), total sleep time (MD −0.69, 95% CI −1.16 to −0.23; 3 studies, 197 participants) and sleep efficiency (MD −0.96, 95% CI −1.38 to −0.54; 3 studies, 197 participants), but may have no effect on perceived sleep interruption (MD −0.53, 95% CI −1.47 to 0.40; 3 studies, 197 participants). In addition, three studies (136 participants) included objective measures of sleep-onset latency, total sleep time, sleep efficiency and sleep interruption and showed that listening to music may not improve these outcomes compared to no treatment or TAU. None of the included studies reported any adverse events. Authors' conclusions: The findings of this review provide evidence that music may be effective for improving subjective sleep quality in adults with symptoms of insomnia. More research is needed to establish the effect of listening to music on other aspects of sleep as well as the daytime consequences of insomnia.