Sleepiness is a status ailment found in all three games in the Mother series. When a character is asleep, they cannot do anything until they wake up. They usually wake up within 2 or 3 turns, or if said character is attacked. Many moves can cause this status ailment, one of the best examples being Hypnosis.
An unfinished attack used by some of the unused bosses known as PK Ω puts the entire party to sleep.