Pokémon: Hypno's Naptime

Our heroes come to a town in which a great deal of children have recently turned up missing. Working together with Jenny to search for these children, they begin an investigation at the Pokémon Center, inquiring into the disappearances, but Joy has other concerns. For some reason, the Pokémon in the Center aren't getting well at all. Jenny reveals that a mysterious sleep wave has been passing in and out of the town for several days. Our heroes suspect a connection between the missing children and the Pokémon's poor condition. Team Rocket begins a sinister search of their own for the source of these strange occurrences. What could be the cause of this sleep wave?