Asleep is a status ailment in the Smash Bros. games which characters enter when they are hit by certain moves, most famously Jigglypuff's Sing attack. In this state, a character is unable to do anything. Rotating the control stick back and forth and button mashing will shorten the effect, waking the character up sooner. In the N64 version, characters sleep longer at lower damages, but in Melee and Brawl, characters sleep longer when they have taken more damage. Characters cannot fall asleep unless they are on the ground; any character in the air, on an edge, or on a ladder are immune to sleep attacks. Sleep ends early once the character is hit by any attack (unless it does not cause flinching) or the ground disappears.
In Super Smash Bros., only Jigglypuff's Sing had this effect. Also, the animation for asleep characters is the same as the animation for when they are stunned, except that they flash from different colors, emit musical notes (as if they were hypnotized by the song), and use a different voice clip, such as Mario snoring like in Super Mario 64 instead of constantly saying "Uuh!". In Melee and Brawl, all characters are given unique animations for this state. For example, Donkey Kong rocks on his back and puts his thumb on his lips, and Fox curls up on the ground like his real-world animal counterpart. Sleeping characters also flash magenta, and they also emit stylised "bubbles" from their face's (with the exception of Mr. Game and Watch, who instead has stylised Z's).
Gallery
Mario sleeping in Smash 64.
Donkey Kong sleeping in Melee.
King Dedede has fallen asleep from Luigi's Final Smash, Negative Zone in Brawl.