Acoustic Stimulation and Other Emerging Approaches to Enhance Sleep: Design Notes for the Next Generation of Closed-loop Neurostimulation Technology

Sleep is indispensable to human health, supporting memory consolidation, emotional regulation, immune function, and metabolic homeostasis. Despite its importance, chronic sleep deprivation is pervasive, with especially high prevalence in operational and clinical populations. This review synthesizes recent advances in sleep enhancement through closed-loop neurostimulation, focusing on systems that dynamically interact with endogenous brain rhythms to improve sleep quality and efficiency. Key oscillatory targets—including slow waves, sleep spindles, and hippocampal ripples—are examined in the context of memory consolidation, with evidence supporting their augmentation via temporally precise auditory and electrical stimulation. Emerging thermal modulation techniques, particularly cyclic skin temperature manipulation, are also highlighted for their capacity to increase slow wave sleep by enhancing the core-to-periphery temperature gradient. Complementary methods targeting rapid eye movement (REM) sleep and sleep onset latency are discussed, underscoring the versatility of closed-loop systems. The review identifies outstanding questions regarding stimulation timing, modality selection, physiological limits, and the dissociation between slow oscillations and delta activity. To address these challenges, the authors advocate for a modular, open-source ecosystem that integrates real-time sleep state decoding with configurable effectors across auditory, electrical, and thermal domains. Such a platform would enable reproducible, scalable, and personalized interventions for sleep enhancement. This systems-level approach aims to accelerate translational research and catalyze a paradigm shift toward actively regulated, on-demand sleep interventions.