Non-invasive electrical stimulation for sleep disturbances in adults: protocol for an evidence-mapping umbrella review of systematic reviews and meta-analyses with subgroup analysis by intervention type and population.

Background: Sleep disturbances affect 10%-30% of adults worldwide. Non-invasive electrical stimulation (e.g., transcranial electrical stimulation) has emerged as a promising non-pharmacological intervention. Although numerous systematic reviews and meta-analyses have been published, they vary considerably in methodological quality, populations, intervention types, and conclusions. No umbrella review has yet synthesised the evidence across different modalities and populations. This evidence mapping umbrella review aims to systematically chart the existing systematic reviews, assess methodological quality, quantify overlap, and describe evidence patterns across diverse modalities and populations. Methods: Following JBI guidelines, we will search PubMed, Embase, Cochrane Database of Systematic Reviews, Web of Science, PsycINFO, and Scopus (inception to April 2026), restricted to English. Grey literature will be searched via PROSPERO, ClinicalTrials.gov, Google Scholar (first 200 records), and reference list screening (snowballing). We will include systematic reviews and meta-analyses of randomised controlled trials evaluating any non-invasive electrical stimulation for sleep outcomes. Two reviewers will independently screen, extract data, and assess methodological quality using AMSTAR 2. Primary study overlap will be quantified by the Corrected Covered Area. Where feasible, we will calculate 95% prediction intervals, perform Egger's regression tests, and conduct excess significance tests using review-level summary estimates. Subgroup analyses will be stratified by intervention and population type. Sensitivity analyses will exclude: (1) reviews with critically low AMSTAR 2 ratings, (2) preprints, (3) reviews at high risk of reporting bias, and (4) studies where sleep is not the primary outcome. The primary outcome is subjective sleep quality; total sleep time is a key secondary outcome. Evidence will be graded using the Fusar-Poli classification, with GRADE for key outcomes. Discussion: This umbrella review will provide the highest level of evidence synthesis, identifying modalities with more consistent or higher certainty evidence and highlighting areas where evidence remains uncertain. Limitations include restriction to English (which may disproportionately impact modalities such as TEAS), expected heterogeneity, and possible insufficient data for some subgroup analyses. All amendments have been documented in PROSPERO (CRD420261357590). Clinical Trial Registration: https://www.crd.york.ac.uk/prospero/, identifier CRD420261357590.