A sleeping draught is a tonic that, induces a powerful slumber on the drinker that lasts through a full night.
Gaius later prepared stronger versions of the elixir as Morgana's nightmares grew more intense, though the draught did nothing to stop the dreams (The Gates of Avalon).
Arthur ingested another sleeping draught, unknowingly, when he volunteered to drink what he believed to be a mixture of water and poison after navigating the Labyrinth of Gedref (The Labyrinth of Gedref).
Gaius decided to give Morgana more and more powerful sleeping draughts while her powers were emerging (The Nightmare Begins).
When the Goblin, while possessing Gaius, stole her bracelet, Morgana asked the court physician to give her a sleeping draught again, but Gaius tricked her and gave her a potion that caused her to suffer from flatulence (Goblin's Gold).