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If the wielder is asleep, its sleep counter decreases by two every turn before it attacks rather than the usual one, allowing the wielder to either wake up immediately or after a single turn, depending on the randomly generated sleep counter. This effect also applies to user-induced sleep caused by moves such as Rest.
Despite being a decent ability, many of the Pokemon who get this ability cannot utilize it well, either because they get a better secondary ability, or because they are just not very good themselves. Sleep-inducing moves are also not very common; the most common is Spore, making this ability fairly unreliable without using Rest. Many Pokemon who do get this ability however are not bulky enough to take a hit to make this an effective strategy.
Found in Dream World