Sleep disturbances are increasingly recognized as early indicators and potentially modifiable contributors to cognitive decline and dementia. This systematic review synthesizes evidence from 31 original studies published between 2015 and 2025, encompassing observational, population-based, and interventional designs. Consistent findings indicate that insomnia and obstructive sleep apnoea (OSA) are associated with increased risk of cognitive decline and dementia, with hazard ratios ranging from 1.36 to 1.84. Mechanistic studies show that insomnia accelerates amyloid-β and tau accumulation through impaired glymphatic clearance and neuroinflammation, while OSA contributes via intermittent hypoxia, oxidative stress, and cerebrovascular dysfunction. Circadian rhythm disturbances, hypersomnia, and REM sleep behaviour disorder (RBD) were also linked to cognitive impairment, particularly non-Alzheimer dementias such as Lewy body and front temporal dementia. Interventional evidence suggests that continuous positive airway pressure (CPAP) and cognitive behavioural therapy for insomnia (CBT-I) improve cognitive outcomes and may mitigate dementia risk. Study quality was appraised using the Newcastle–Ottawa Scale and Cochrane RoB-2 tools, and overall certainty of evidence was evaluated using the GRADE framework, indicating low-to-moderate confidence 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 to confirm causality and strengthen the evidence base for sleep-focused dementia risk reduction.