Sleep is an attack effect that has appeared in every game in the Super Smash Bros. series. Fighters hit by a sleep attack fall asleep for an amount of time based on the victim's total damage and the knockback of the move used. If a fighter that is already asleep is hit by a sleep attack, its current sleep condition will not be increased in duration or reset.
Since enemies are immune to all status conditions, sleep attacks instead trip Smash Run enemies. In Super Smash Bros. Ultimate, the Support Spirit skill Slumber Immunity prevents the equipped fighter from being put to sleep.
The formula for the maximum sleep duration in frames in Ultimate is 25k + 0.65p + 40, where k is the knockback the sleeping move would have dealt and p is the target's percent (or in Stamina battles, how much of their max HP they've lost).
List of sleep attacks
By enemies and bosses
By stage hazards
The Spirit Battle condition "The floor is sleep-inducing" creates a hazard that causes players that touch it to fall asleep.