Review of VLPO subregion definitions and translational challenges in sleep research

The ventrolateral preoptic nucleus (VLPO) of the anterior hypothalamus is a major sleep-promoting region and a key component of the networks governing sleep–wake regulation. Rather than acting as a unitary “sleep center,” the VLPO is increasingly understood as a molecularly and functionally heterogeneous ensemble embedded within distributed preoptic, hypothalamic, and brainstem circuits. In this review, we revisit classical lesion, c-Fos, and electrophysiological evidence alongside recent advances in single-cell transcriptomics, projection-specific circuit mapping, and in vivo functional interrogation. We summarize how the core and extended VLPO and their constituent neuronal populations—including GABA/galanin co-expressing neurons, non-galanin GABAergic neurons, and glutamatergic neurons—contribute differentially to non-rapid eye movement (NREM) sleep, rapid eye movement (REM)-associated processes, state transitions, thermoregulation, and metabolic integration. We further examine how circadian timing, homeostatic sleep pressure, metabolic signals, and thermal inputs converge on VLPO-related circuitry through interactions with neighboring preoptic populations and broader hypothalamic–brainstem arousal systems. This network perspective helps reconcile classical VLPO-centered models with emerging evidence for distributed and partially redundant control of sleep–wake states. We also highlight unresolved issues, including the molecular definition of VLPO subregions, functional overlap with adjacent preoptic nuclei, inconsistencies across experimental approaches, and the challenge of translating rodent circuit findings to humans. Taken together, current evidence supports viewing the VLPO not as an isolated sleep center, but as a multisystem integrator within distributed sleep-regulatory networks.