Jerome Siegel, a professor of psychiatry at the University of California, Los Angeles, argues that sleep evolved to optimize animals’ use of time, keeping them

Jerome Siegel, a professor of psychiatry at the University of California, Los Angeles, argues that sleep evolved to optimize animals’ use of time, keeping them safe and hidden when the hunting, fishing or scavenging was scarce and perhaps risky. IIRC a character from "Beggars in Spain" (a Nancy Kress novella that she later expanded into a trilogy) makes a similar argument for why her clever genetic hack for creating children who never need to sleep will have no negative side effects. It seemed like an audacious handwave when I read it; I had no idea it had the backing of a Real Scientist.