<p>What is the purpose of sleep? This ubiquitous biological phenomenon has retained a frustratingly opaque primordial function, despite decades of neurocentric research. There are multiple competing hypotheses but no unifying framework, partly because the field has sought answers in the brain that may be much deeper. In this perspective, we argue that sleep did not evolve as a neural phenomenon but as a conserved cellular state based on an ancient metabolic enzyme network. We mapped orthologs of sleep genes across the tree of life (ToL), from the Last Universal Common Ancestor (LUCA) to bacteria, archaea, protists, fungi, plants, and animals, and identified a core set of universally conserved enzymes involved in amino acid metabolism, protein synthesis, detoxification, and response to oxidative stress. Sleep-like processes exist in some lineages that do not have nervous systems, so some form of sleep probably existed before multicellularity. We suggest that sleep was originally a primordial cellular strategy for metabolic homeostasis and repair, which was later co-opted and elaborated by complex physiological systems. This phylogenomic perspective redefines sleep as a key feature of biological existence, offering new perspectives on the understanding of sleep disorders, metabolism, and evolutionary medicine.</p>