"Sleep to gradually recover Health! You'll sleep for as long as you keep pressing the button."
— Dragon Ball Xenoverse in-game description
Unlike most techniques, Sleep is a basic biological function of most living beings (including Earthlings, Saiyans, Majins, most alien races, and even deities) and, as a result, it is one of the few techniques that can be used by anyone regardless of power level (even untrained persons like Bulma).
Video Game Appearance
In the Budokai Tenkaichi series, the technique is used by Grandpa Gohan, Great Ape Kid Goku, Great Ape Raditz, Cui, Frieza Soldier, Goten, Majin Buu, and Kid Buu. The character falls asleep for a few seconds, during which they are very vulnerable to attacks. If it is successful, the character's health and ki will recover substantially, along with adding all stat bonuses.
In Dragon Ball Xenoverse, it appears under the name Quick Sleep and is a Super Skill that allows the user to gradually recover health for as long as they remain asleep. It appears as one of Kid Buu's Super Skills in his 1st skillset. It can be also be obtained by the Future Warrior as a reward in Parallel Quest 41: "Frieza's Nightmare Returns!", however it is can only be used by the Future Warrior if they are a Majin.
In Dragon Ball Xenoverse 2, Quick Sleep returns as one of Kid Buu's Super Skills in his Pure 2 Skillset and can be obtained by the Future Warrior by purchasing it from the Skill Ship in Conton City after defeating Mira (Final Form) in the main story. Like in Xenoverse, it can only be used by Majins. Additionally, it also appears as one of the Super Skills available to a Majin Future Warrior when they are transformed into their Pure Majin form by the Purification Awoken Skill (as it basically grants them access to Kid Buu's abilities and techniques).