The hormone that regulates sleep and other circadian processes in vertebrates also controls night-time behaviour in zooplankton, suggesting early evolutionary origins for the hormone.

Melatonin is produced by many organisms, but its function in invertebrates has not been clear. Maria Antonietta Tosches, Detlev Arendt and their colleagues at the European Molecular Biology Laboratory in Heidelberg, Germany, studied larvae of the marine worm Platynereis dumerilii, which move up and down in the water at certain times of the day. The authors found that the larvae make melatonin in the brain and that production ramps up at night. This boosted neuronal activity, which resulted in less swimming, allowing the larvae to drift downward.

Melatonin evolved in early animals to coordinate their behaviour with the time of day, the authors propose.

Cell 159, 46–57 (2014)