Objectives: To quantify the effects of melatonin on total sleep time and global cognition, and to examine secondary outcomes (sleep efficiency, circadian markers, neuropsychiatric symptoms, depression, activities of daily living) in older adults with cognitive impairment.

Design: Systematic review and meta-analysis of randomised controlled trials (PROSPERO CRD42024539227).

Data sources: PubMed, PsycINFO (via EBSCOhost), CINAHL Plus, Cochrane Library, Web of Science, to October 2024.

Eligibility criteria: Randomised controlled trials (RCTs) comparing melatonin versus placebo in adults ≥65 years with mild cognitive impairment or dementia.

Data extraction and synthesis: Two reviewers extracted data; risk of bias was assessed by RoB 2. Random-effects models estimated Hedges' g and mean differences (MD), with Benjamini-Hochberg false discovery rate (FDR) correction for multiple outcomes. Heterogeneity assessed via I2; publication bias via Egger's test.

Results: Ten RCTs (n = 516) were included. Melatonin increased total sleep time by MD +12.4 min (95% CI 8.1-16.7; P < .001; I2 = 72%; g = 0.42) and improved Mini-Mental State Examination scores by MD +1.8 points (95% CI 0.9-2.7; P = .002; I2 = 65%; g = 0.47). Neuropsychiatric symptoms decreased (g = -0.23; 95% CI -0.44 to -0.03; P = .02). Effects on sleep efficiency, circadian markers, depression and activities of daily living were small and nonsignificant.

Conclusions: Melatonin yields modest improvements in sleep duration and cognition that approach clinically important thresholds but fall short of definitive efficacy. Larger, rigorously designed RCTs with harmonised outcomes and pre-specified primary endpoints are needed to confirm these findings.