When Kirby inhales a lethargic Noddy, he instantly falls asleep and can't do anything until he wakes up. This nap takes approximately 5 seconds, depending on the game. Since the player is unable to willingly drop the ability, Sleep has no Helper in Kirby Super Star/Ultra, like the other limited-use abilities such as Mike and Crash. Sleep's Copy Essence is also occasionally used as a hazard in some games due to the nature of the ability.
Sleep isn't always completely useless, however. In Kirby's Return to Dream Land, it gained the power to deal minor damage with a popped sleep bubble when Kirby wakes up. In Kirby Air Ride, the victim ailed with the ability is able to pass it on to another racer by crashing into them, leaving them with the debilitating ability instead. Also, if the player obtains the Sleep Ability Scroll in Kirby: Squeak Squad, Kirby will slowly regain health as he sleeps, up to just under 25% with all 8 Vitality pieces collected, so a Sleep bubble heals more health than common food outside of combat, but Kirby is still defenseless while using it. It is possible to sleep on the world map, which can be done by exiting the stage while sleeping or tapping a Sleep ability bubble on the map. It's also possible to exit the stage while sleeping, thus falling asleep again on the world map, and subsequently regenerating health again. With this method, Kirby can regain almost 50% of his max HP.
Sleep appeared with a full moveset for the first time in Kirby Battle Royale. It was added as a free DLC ability on February 1, 2018 after winning the second 25th Anniversary Poll. In this game, Sleep Kirby always carries a pillow, and his attacks are partially inspired by pillow fighting. His animations suggest that he is sleepwalking the entire time while fighting. While Kirby still falls asleep if left idle, the player can instantly break out of it at any time; the sleeping state also causes Kirby to slowly regain health like in Kirby: Squeak Squad. Sleep's moveset in Kirby Battle Royale is defined by pillow strikes that are quick and hard-hitting, but have limited range. It also features a much more powerful and effective variant of the Early Wake sleep bubble attack.
In most games, Sleep Kirby appears at the game over screen (sometimes without the hat). If the player chooses to continue, Sleep Kirby will be woken up (a different method depending on the game), then Kirby will run off-screen (or towards the screen in Kirby: Triple Deluxe and Kirby: Planet Robobot) and the game continues from the beginning of the level. If the player chooses to quit, however, Sleep Kirby will inhale the hand that is interfering with his sleep and will continue sleeping. (However, in Kirby's Dream Land, the hand will charge up and punch away the sleeping Kirby, while in Kirby Super Star and Kirby Super Star Ultra, the hand puts Kirby on the moon or Planet Popstar, gives him a blanket, then leaves.)
Kirby falls asleep for about five seconds, leaving him completely defenseless. When he wakes up this power disappears.
Kirby spins around in the air with his pillow held out, until he slams it down on the ground.
65 per hit
Kirby builds up a large snot bubble that travels slowly across a long distance, popping upon impact.
Kirby pulls out a green nightcap and drops it on an opponent's head, putting them to sleep and leaving them vulnerable to attack..
Don't move for a while
Kirby falls asleep and dreams of eating cake. Gradually restores health, and the player can end it at any time by moving or attacking. Normally, Kirby will rest his head on top of the pillow, but if performed after Pillow Takedown or Spinning Pillow Dive, Kirby will leave his pillow outstretched in front of him.
Sleep appears in Kirby: Right Back at Ya! in episode 16, Flower Power. King Dedede tricks Kirby into inhaling a Noddy and Kirby falls asleep. In the anime, Sleep Kirby is rather different than in the games, as it doesn't give Kirby a hat, and it was supposed to make Kirby sleep for 100 years, whereas it only lasts seconds in the games. He would only wake up if he smelt the scent of the legendary Pukey Flower.
Sleep Kirby also appears as a collectible trophy in Super Smash Bros. Brawl and Super Smash Bros. for Wii U.
One of Kirby's idle poses in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U involves him falling asleep for a brief second, then waking himself up.
...Please... ...Let me sleep... ...A little more... ...Please...
Just take a little break and let Kirby sleep!
-DO NOT DISTURB- I need my beauty rest.
Just a li'l while longer... ...Please. Let me sleep...
Just...five more...minutes. Please...just let me sleep. If you must...wake me... Just...shake me... Zzz...zzz...zzz...
Just...three more minutes, Mom. I'm just so...yawwwwn...sleepy. If you gotta...wake me up... Just...mash the...+Control Pad. Zzzzzz... Zzzzzzz...
Sleep is definitely important, but it can be dangerous if you drop off in the wrong place! Luckily, you can wake up quickly by button mashing.
Doze off to fight back! This sleepy-time ability will surprise your rivals!
Zzz... Just a...few more...minutes... ...Please...I mean...just lemme sleep... Zzz...If you need...to wake me up... ...just ask...a friend...to...hit...me...zzz...
Kirby after he copies the ability to sleep peacefully. Kirby takes on this form when he inhales Noddy or other such enemies. Kirby cannot move at all while sleeping, so extra care should be taken not to use this ability when enemies are near. Kirby, in his slumber, tends to drool, and in his panicked awakening, isn't it cute how he forgets to wipe off the drool?”
You don't get to see sleeping heroes very often. They're so busy rescuing princesses or saving the world, they just don't have time for a little nap. Not so with Kirby! The moment he inhales the snooze-prone enemy Noddy, he pops on his sleeping cap and dozes right off. Ah, Kirby, what dreams might you be having today?”
Throw your rivals off guard with your drowsy demeanor. Let your instincts do the fighting for you while you sleep.”
— Official Kirby Battle Royale Website
Sleep was technically the first ability to get a hat. The hat can be seen in the Kirby's Adventure intro from the first picture. It is Kirby's sleeping cap when he wakes up from his bad night's sleep.
Sleep is available in Grape Garden's Museum, although the ability is quite useless and indeed cannot even be taken out of the Museum.
Sleep is one of the very few Copy Abilities that can be obtained via inhaling in Milky Way Wishes. Although a copy pedestal for Sleep is seen in other sub-games, in Kirby Super Star and it's remake, it is not used as a Copy Essence Deluxe here.
In Kirby Air Ride, the Sleep ability is quite similar to the Dizzy Virus from Mario Kart Arcade GP.
In Kirby Air Ride, and Kirby's Return to Dream Land onwards, Kirby's sleep hat appears purple with yellow polka dots instead of the traditional green and white polka dot hat.
In the "Staff Credits" cutscene in Kirby Super Star Ultra and the intro of Kirby: Triple Deluxe, Kirby's Sleep hat has yellow stars instead of white dots.
In the 25th Anniversary Poll that ran on the Official Kirby Website, Sleep was unveiled on November 7, 2017 to have been the third place winner. However, Sleep won the second poll, making it the second ability to be added to Kirby Battle Royale.
Kirby Battle Royale is, thus far, the only Kirby game where Sleep is optimized to be a truly usable Copy Ability.
Like Mirror, Sleep lacks any alternate headgear in Kirby Battle Royale. Since both abilities were added to the game through patches later on, this could be a result of the DLC content being developed with more limited resources.
Sleep is able to heal in only Kirby: Squeak Squad and Kirby Battle Royale. It is also one of two abilities able to regain health in Kirby Battle Royale; the other is Doctor.
On most game over screens, Kirby can be sleeping.
Unlike most of the flavor texts of other Copy Abilities, Sleep's flavor texts are almost always narrated by Kirby himself.
In artwork from Kirby's Adventure, Sleep Kirby is shown to be flattened rather than slumped like in-game.