Dual-Resonance Shapes REM Sleep: A Mechanistic Link Between Homeostasis, Circadian Phase, and Sleep Deficit.

How circadian and homeostatic processes interact to shape REM sleep architecture remains unresolved. Here, we empirically confirm the existence of a conserved resonance curve governing REM episode duration, as previously predicted by the wave model of sleep dynamics. This curve emerges from two converging resonance states - homeostatic and circadian - that form a stable framework across conditions. Their relative phase and amplitude determine REM structure, producing bell-shaped, linear, or bimodal profiles. The model reproduces REM dynamics during regular sleep, extension, and post-deprivation recovery. Crucially, REM duration alone provides an experimentally accessible readout: simultaneously revealing circadian phase and quantifying sleep deficit as additional hours needed to restore homeostatic equilibrium. Together, these findings reposition REM duration from a descriptive to a diagnostic marker, and a mechanistic signal, linking sleep architecture to the dynamic interplay of internal regulatory systems. Summary sentenceREM episode duration emerges as a dual readout of circadian phase and sleep deficit, reflecting the interaction of homeostatic and circadian resonance processes.