When Magic Booster is used in the 2013 PC re-release, the player's inventory gains 100 Sleep spells, among other basic spells.
Sleep can be drawn from many enemies at all levels, and can be refined from Sleep Powders using the Guardian Force Diablos' ability ST Mag-RF. Funguar and Grat are great sources of Sleep, as not only can Sleep be drawn from them, but they are sources of Sleep Powder, which can be refined.
Sleep when cast will inflict the Sleep status effect on a target that is not immune to it. This prevents the target from acting, for approximate 13 seconds, until hit with a physical attack, rendering them vulnerable to magic attacks. It also extends the Protect, Shell, and Regen statuses for Sleep's duration. It has no effect on a target's Evasion, so they may still evade attacks.
Sleep can be cast using the Magic command, Selphie's Slot Limit Break, or Rinoa's Angel Wing if she has it in her inventory. Casting Sleep in battle increases compatibility with Doomtrain by 1 and with Siren by 0.6, but lowers compatibility with Cerberus and Carbuncle by 0.2.
Sleep can be inflicted on enemies with a physical attack when junctioned to ST-Atk-J, allowing a character with Sleep junctioned to permanently inflict Sleep against enemies, effectively preventing them from ever acting while the character continues to inflict the status. As a large variety of enemies, and even some bosses early on, are vulnerable to Sleep, making it one of the best choices for status effects to junction to ST-Atk-J. Though other members of the party using physical attacks will wake the enemy, the ability to constantly re-apply Sleep will still prevent the enemy from acting most of the time. As a tutorial to Status Attack, Quistis Trepe advises the player to junction Sleep to use against T-Rexaur, which then becomes helpless against a barrage of Blizzard spells and Shiva.
Sleep is outclassed by Stop, as it can also be junctioned to ST-Atk-J and the enemy will not wake up after being hit with physical attacks, meaning the player can permanently stop enemies vulnerable to it. However, Stop is harder to obtain than Sleep, making Sleep a solid choice in absence of Stop.
Additionally, Sleep can be junctioned to ST-Def-J to provide complete resistance to it when 100 Sleeps are junctioned. This can be vital against enemies that inflict the status effect, such as Malboros, as Sleep renders a character vulnerable to magic attacks, making it important to defend against.
Due to the ability to junction Sleep to ST-Atk-J or ST-Def-J, it is better to stock Sleeps rather than cast them.