He had once before put Zeus to sleep at the bidding of Hera, allowing her to cause Heracles (who was returning by sea from Laomedon's Troy) great misfortune.
Hipnos hizo dormir anteriormente a Zeus una vez a instancias de Hera, lo que le permitió causar grandes infortunios a Heracles (quien regresaba por mar de la Troya de Laomedonte).