Sleep Disorders as Predictors of Cognitive Decline and Dementia: A Systematic Review

Abstract: Sleep disturbances are increasingly recognized as early indicatorsand potentially modifiable contributors to cognitive decline and dementia.This systematic review synthesizes evidence from 31 original studiespublished between 2015 and 2025, encompassing observational, populationbased,and interventional designs. Consistent findings indicate that insomniaand obstructive sleep apnoea (OSA) are associated with increased risk ofcognitive decline and dementia, with hazard ratios ranging from 1.36 to 1.84.Mechanistic studies show that insomnia accelerates amyloid-β and tauaccumulation through impaired glymphatic clearance andneuroinflammation, while OSA contributes via intermittent hypoxia,oxidative stress, and cerebrovascular dysfunction. Circadian rhythmdisturbances, hypersomnia, and REM sleep behaviour disorder (RBD) werealso linked to cognitive impairment, particularly non-Alzheimer dementiassuch as Lewy body and front temporal dementia. Interventional evidencesuggests that continuous positive airway pressure (CPAP) and cognitivebehavioural therapy for insomnia (CBT-I) improve cognitive outcomes andmay mitigate dementia risk. Study quality was appraised using theNewcastle–Ottawa Scale and Cochrane RoB-2 tools, and overall certainty ofevidence was evaluated using the GRADE framework, indicating low-tomoderateconfidence in current findings. This review provides an updated,integrative synthesis highlighting sleep disorders as biologically plausible,clinically actionable, and underutilized targets for dementia prevention.Future large-scale, biomarker-based randomized trials are essential toconfirm causality and strengthen the evidence base for sleep-focuseddementia risk reduction.Keywords: Sleep disorders; Insomnia; Obstructive sleep apnoea; Cognitivedecline; Dementia; REM sleep behaviour disorder; Circadian rhythm;Neurodegeneration