A fruit fly broadcasts how badly it needs sleep while it is still awake. Two mutually inhibitory half-centers of sleep-control neurons (dorsal fan-shaped body) trade UP and DOWN states, so the circuit hums a slow alternating rhythm all day. Underneath sits a biochemical memory: mitochondrial history written into Shaker–Hyperkinetic channel complexes sets how loud the rhythm gets. Replay the rhythm artificially and the fly falls asleep — the rhythm isn't a readout, it's a lever. (Hasenhuetl, Sarnataro & Miesenböck, Nature Neuroscience, Oct 15 2026 — this piece is a labelled toy model, not their data.)