Light affects sleep/wake behaviors indirectly by providing a cue that entrains circadian rhythms but also via direct regulation of behavior through a phenomenon known as masking. While circadian entrainment by light is well characterized at the molecular level, genes that underlie masking are largely unknown. Using zebrafish, a diurnal vertebrate, we found that both overexpression and mutation of the neuropeptide prokineticin 2 (Prok2) affect sleep/wake behaviors in a light-dependent but circadian-independent manner. We also found that the Prok2 overexpression phenotype requires prokineticin receptor 2 and that Prok2 overexpression induces expression of galanin, a known sleep-inducing peptide, in the hypothalamus in a light-specific manner. These results suggest a novel genetic and neuronal circuit that links light via Prok2 to a known sleep-promoting center.