Sleep. How can such an evolutionarily detrimental trait be so deeply conserved?

This question has haunted neuroscience for decades. Sleep demands stillness, vulnerability, and precious time that could be spent feeding, mating, or evading predators. By all accounts, evolution should have eliminated it — yet sleep persists in nearly every brain, from insects to humans.

The SPIN model provides a clear answer: sleep is not just restorative; it is structurally essential for preserving the integrity of synaptic networks in highly plastic brains. Without sleep, connections between neurons weaken naturally due to constant spontaneous firing and the physics of synaptic decay. Waking experiences and REM sleep selectively strengthen important circuits, but over time this is not enough to maintain the entire network’s coherence.

Slow-wave sleep (SWS) solves this fundamental problem. During SWS, global synchrony sweeps through the cortex, reinforcing even weak or dormant connections that would otherwise be lost. This non-specific, system-wide reactivation acts as a structural safeguard — stabilizing the brain’s architecture while still allowing flexibility, learning, and the pruning of truly unused connections.

In this way, SPIN reframes sleep not as a beneficial add-on but as an evolutionary necessity. It explains memory stability, sparse coding, age-related loss of plasticity, and even points toward more resilient artificial networks that mimic how the brain preserves itself. Sleep persists because, without it, the network collapses.

This highly exaggerated figure illustrates how synaptic strength evolves for two example synapses across sequential cycles of wakefulness, REM sleep, and slow-wave sleep (SWS). Synapse A (green) represents an actively used connection: it is strengthened during wakefulness through real-world experience and further reinforced by selective reactivation during REM sleep. Each SWS phase globally stabilizes both strong and weak connections, preserving the network’s structural integrity. In contrast, Synapse B (red) represents an unused connection: it passively decays during wake and REM phases, receiving only partial stabilization during SWS. This demonstrates that SWS alone slows synaptic decay but cannot fully maintain unused connections without selective reactivation. Phase shading indicates Wake (yellow), REM (orange), and SWS (blue). The model shows how the interplay of selective and global mechanisms preserves meaningful circuits while allowing unused connections to fade, supporting sparse, energy-efficient coding.